diff --git a/.gitignore b/.gitignore
index faeae3a..82c57bf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ build/app.dill
 .dart_tool/pub/bin/test/test.dart.snapshot.dart2
 .packages
 pubspec.lock
+.dart_tool
diff --git a/lib/src/grammar.dart b/lib/src/grammar.dart
index 73efaad..86cabe9 100644
--- a/lib/src/grammar.dart
+++ b/lib/src/grammar.dart
@@ -1,6 +1,6 @@
 import 'dart:core';
 
-var grammar = {
+var grammar = <String, dynamic>{
   'v': [
     {'name': 'version', 'reg': r'^\d$'}
   ],
@@ -90,8 +90,10 @@ var grammar = {
       'format': (o) {
         return (o['encoding'] != null)
             ? 'rtpmap:%d %s/%s/%s'
-            : (o['rate'] != null) ? 'rtpmap:%d %s/%s' : 'rtpmap:%d %s';
-      }
+            : (o['rate'] != null)
+                ? 'rtpmap:%d %s/%s'
+                : 'rtpmap:%d %s' ;
+      }as dynamic
     },
     {
       // a=fmtp:108 profile-level-id=24;object=23;bitrate=64000
@@ -114,7 +116,7 @@ var grammar = {
       'names': ['port', 'netType', 'ipVer', 'address'],
       'format': (o) {
         return (o['address'] != null) ? 'rtcp:%d %s IP%d %s' : 'rtcp:%d';
-      }
+      } as dynamic
     },
     {
       // a=rtcp-fb:98 trr-int 100
@@ -130,7 +132,7 @@ var grammar = {
       'names': ['payload', 'type', 'subtype'],
       'format': (o) {
         return (o['subtype'] != null) ? 'rtcp-fb:%s %s %s' : 'rtcp-fb:%s %s';
-      }
+      }as dynamic
     },
     {
       // a=extmap:2 urn:ietf:params:rtp-hdrext:toffset
@@ -143,7 +145,7 @@ var grammar = {
             (o['direction'] != null ? '/%s' : '%v') +
             ' %s' +
             (o['config'] != null ? ' %s' : '');
-      }
+      }as dynamic
     },
     {
       // a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32
@@ -154,7 +156,7 @@ var grammar = {
         return (o['sessionConfig'] != null)
             ? 'crypto:%d %s %s %s'
             : 'crypto:%d %s %s';
-      }
+      }as dynamic
     },
     {
       // a=setup:actpass
@@ -254,7 +256,7 @@ var grammar = {
         str += (o['network-id'] != null) ? ' network-id %d' : '%v';
         str += (o['network-cost'] != null) ? ' network-cost %d' : '%v';
         return str;
-      }
+      }as dynamic
     },
     {
       // a=end-of-candidates (keep after the candidates line for readability)
@@ -286,8 +288,8 @@ var grammar = {
             str += ':%s';
           }
         }
-        return str;
-      }
+        return str ;
+      } as dynamic
     },
     {
       // a=ssrc-group:FEC 1 2
@@ -332,7 +334,7 @@ var grammar = {
         return (o['maxMessageSize'] != null)
             ? 'sctpmap:%s %s %s'
             : 'sctpmap:%s %s';
-      }
+      }as dynamic
     },
     {
       // a=x-google-flag:conference
@@ -347,7 +349,7 @@ var grammar = {
       'names': ['id', 'direction', 'params'],
       'format': (o) {
         return (o['params'] != null) ? 'rid:%s %s %s' : 'rid:%s %s';
-      }
+      }as dynamic
     },
     {
       // a=imageattr:97 send [x=800,y=640,sar=1.1,q=0.6] [x=480,y=320] recv [x=330,y=250]
@@ -364,7 +366,7 @@ var grammar = {
       'names': ['pt', 'dir1', 'attrs1', 'dir2', 'attrs2'],
       'format': (o) {
         return 'imageattr:%s %s %s' + (o['dir2'] != null ? ' %s %s' : '');
-      }
+      }as dynamic
     },
     {
       // a=simulcast:send 1,2,3;~4,~5 recv 6;~7,~8
@@ -382,7 +384,7 @@ var grammar = {
       'names': ['dir1', 'list1', 'dir2', 'list2'],
       'format': (o) {
         return 'simulcast:%s %s' + (o['dir2'] != null ? ' %s %s' : '');
-      }
+      }as dynamic
     },
     {
       // old simulcast draft 03 (implemented by Firefox)
diff --git a/lib/src/parser.dart b/lib/src/parser.dart
index 447d3e2..1e3b3c1 100644
--- a/lib/src/parser.dart
+++ b/lib/src/parser.dart
@@ -1,8 +1,15 @@
 import './grammar.dart' show grammar;
 import 'dart:convert';
 
-dynamic toIntIfInt(v) {
-  return v != null ? int.tryParse(v) != null ? int.parse(v) : v : null;
+/// Tries to parse a String as an int.
+/// If the [value] isn't an int then [value] is returned.
+/// If [value] is a null then null is returned.
+dynamic toIntIfInt(String? value) {
+  return value != null
+      ? int.tryParse(value) != null
+          ? int.parse(value)
+          : value
+      : null;
 }
 
 void attachProperties(Iterable<RegExpMatch> match,
@@ -75,11 +82,11 @@ Map<String, dynamic> parse(String sdp) {
         medias.add(media);
       }
       if (grammar[type] != null) {
-        for (var j = 0; j < grammar[type].length; j += 1) {
-          var obj = grammar[type][j];
+        for (var j = 0; j < grammar[type]!.length; j += 1) {
+          var obj = grammar[type]![j];
           if (obj['reg'] == null) {
             if (obj['name'] != null) {
-              location[obj['name']] = content;
+              location[obj['name']! as String] = content;
             } else {
               print("trying to add null key");
             }
@@ -91,13 +98,13 @@ Map<String, dynamic> parse(String sdp) {
               parseReg(obj, location, content);
               return;
             }
-          } else if (RegExp(obj['reg']).hasMatch(content)) {
+          } else if (RegExp(obj['reg']! as String).hasMatch(content)) {
             parseReg(obj, location, content);
             return;
           }
         }
         if (location['invalid'] == null) {
-          location['invalid'] = List();
+          location['invalid'] = <Map<String, dynamic>>[];
         }
         Map tmp = createMap();
         tmp['value'] = content;
@@ -125,7 +132,7 @@ Map<dynamic, dynamic> parseParams(String str) {
       value = line.substring(idx + 1, line.length).trim();
     }
 
-    assert(key != null);
+    // assert(key != null);
     params[key] = toIntIfInt(value);
   });
   return params;
@@ -135,10 +142,11 @@ List<String> parsePayloads(str) {
   return str.split(' ');
 }
 
-List<String> parseRemoteCandidates(String str) {
+/// Returns a list that can contain ints, Strings.
+List<dynamic> parseRemoteCandidates(String str) {
   var candidates = [];
-  List<String> parts = List();
-  str.split(' ').forEach((dynamic v) {
+  List<dynamic> parts = <dynamic>[];
+  str.split(' ').forEach((String v) {
     dynamic value = toIntIfInt(v);
     if (value != null) {
       parts.add(value);
@@ -152,12 +160,12 @@ List<String> parseRemoteCandidates(String str) {
 }
 
 List<Map<String, dynamic>> parseImageAttributes(String str) {
-  List<Map<String, dynamic>> attributes = List();
+  var attributes = <Map<String, dynamic>>[];
   str.split(' ').forEach((item) {
     Map<String, dynamic> params = createMap();
     item.substring(1, item.length - 1).split(',').forEach((attr) {
       List<String> kv = attr.split(new RegExp(r'=').pattern);
-      assert(kv[0] != null);
+      assert(kv.length >= 2);
       params[kv[0]] = toIntIfInt(kv[1]);
     });
     attributes.add(params);
@@ -170,9 +178,9 @@ Map<String, dynamic> createMap() {
 }
 
 List<dynamic> parseSimulcastStreamList(String str) {
-  List<dynamic> attributes = List();
+  List<dynamic> attributes = <dynamic>[];
   str.split(';').forEach((stream) {
-    List scids = List();
+    List scids = <dynamic>[];
     stream.split(',').forEach((format) {
       var scid, paused = false;
       if (format[0] != '~') {
diff --git a/lib/src/writer.dart b/lib/src/writer.dart
index 4b2e39e..e8d0008 100644
--- a/lib/src/writer.dart
+++ b/lib/src/writer.dart
@@ -63,7 +63,7 @@ var defaultOuterOrder = [
 
 var defaultInnerOrder = ['i', 'c', 'b', 'a'];
 
-String write(Map<String, dynamic> session, Map<String, dynamic> opts) {
+String write(Map<String, dynamic> session, Map<String, dynamic>? opts) {
   opts = opts ?? {'outerOrder': null, 'innerOrder': null};
 
   // ensure certain properties exist
@@ -86,7 +86,7 @@ String write(Map<String, dynamic> session, Map<String, dynamic> opts) {
 
   // loop through outerOrder for matching properties on session
   outerOrder.forEach((type) {
-    grammar[type].forEach((obj) {
+    grammar[type]!.forEach((obj) {
       if (obj['name'] != null && session[obj['name']] != null) {
         sdp.add(makeLine(type, obj, session));
       } else if (obj['push'] != null && session[obj['push']] != null) {
@@ -99,9 +99,9 @@ String write(Map<String, dynamic> session, Map<String, dynamic> opts) {
 
   // then for each media line, follow the innerOrder
   session['media'].forEach((mLine) {
-    sdp.add(makeLine('m', grammar['m'][0], mLine));
+    sdp.add(makeLine('m', grammar['m']![0], mLine));
     innerOrder.forEach((type) {
-      grammar[type].forEach((obj) {
+      grammar[type]!.forEach((obj) {
         if (obj['name'] != null && mLine[obj['name']] != null) {
           sdp.add(makeLine(type, obj, mLine));
         } else if (obj['push'] != null && mLine[obj['push']] != null) {
diff --git a/pubspec.yaml b/pubspec.yaml
index 972ee53..e35d8c4 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,13 +1,14 @@
 name: sdp_transform
-version: 0.2.0
+version: 0.3.0
 authors: 
 - cloudwebrtc <duanweiwei1982@gmail.com>
 - rlsutton1 <rlustton1@optusnet.com.au>
+- bsutton <bsutton@noojee.com.au>
 - https://github.com/clux/sdp-transform
 description: Dart implementation of sdp-transform
 homepage: https://github.com/cloudwebrtc/dart-sdp-transform
 environment:
-  sdk: '<3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dev_dependencies:
-  test: ^1.9.3
\ No newline at end of file
+  test: ^1.16.8
\ No newline at end of file
diff --git a/test/regexp_test.dart b/test/regexp_test.dart
index fb697c2..f49ebdb 100644
--- a/test/regexp_test.dart
+++ b/test/regexp_test.dart
@@ -13,7 +13,7 @@ basictest() {
   String str = "Parse my string";
   Iterable<Match> matches = exp.allMatches(str);
   matches.forEach((m) {
-    String match = m.group(0);
+    String match = m.group(0)!;
     print('value = ' + match);
   });
 }
@@ -23,9 +23,9 @@ basictest2() {
   String str = "o=- 20518 0 IN IP4 203.0.113.1";
   Iterable<Match> matches = exp.allMatches(str);
   matches.forEach((m) {
-    print('group(0) = ' + m.group(0));
+    print('group(0) = ' + m.group(0)!);
     for (var i = 0; i < m.groupCount; i++)
-      print('group(' + (1 + i).toString() + ') = ' + m.group(i + 1));
+      print('group(' + (1 + i).toString() + ') = ' + m.group(i + 1)!);
   });
 }
 
@@ -51,7 +51,7 @@ basictest3() {
   ];
   Iterable<Match> matches = exp.allMatches(str);
   matches.forEach((m) {
-    print('group(0) = ' + m.group(0));
+    print('group(0) = ' + m.group(0)!);
     for (var i = 0; i < m.groupCount; i++)
       if (m.group(i + 1) != null)
         print('group(' +
@@ -59,7 +59,7 @@ basictest3() {
             ') ' +
             names[i] +
             ' = ' +
-            m.group(i + 1));
+            m.group(i + 1)!);
   });
 }
 
@@ -85,7 +85,7 @@ basictest4() {
   Iterable<Match> matches = exp.allMatches(str);
   matches.forEach((m) {
     print('group(0) = ' +
-        m.group(0) +
+        m.group(0)! +
         ", groupCount = " +
         m.groupCount.toString());
     for (var i = 0; i < m.groupCount; i++)
@@ -95,6 +95,6 @@ basictest4() {
             ') ' +
             names[i] +
             ' = ' +
-            m.group(i + 1));
+            m.group(i + 1)!);
   });
 }
diff --git a/test/sdp_parse_test.dart b/test/sdp_parse_test.dart
index 6a1d575..168cb56 100644
--- a/test/sdp_parse_test.dart
+++ b/test/sdp_parse_test.dart
@@ -41,14 +41,14 @@ deepEqual(dynamic value, dynamic match, String message) {}
 
 _testAsterisk() async {
   String sdp =
-      await new File('./tests/sdp_test_data/asterisk.sdp').readAsString();
+      await new File('./test/sdp_test_data/asterisk.sdp').readAsString();
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
 }
 
 _testNormalSdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/normal.sdp').readAsString();
+      await new File('./test/sdp_test_data/normal.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -205,7 +205,7 @@ _testNormalSdp() async {
  * ignore the name
  */
 _testHackySdp() async {
-  String sdp = await new File('./tests/sdp_test_data/hacky.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/hacky.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -323,7 +323,7 @@ _testHackySdp() async {
 
 _testIceliteSdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/icelite.sdp').readAsString();
+      await new File('./test/sdp_test_data/icelite.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -336,7 +336,7 @@ _testIceliteSdp() async {
 
 _testInvalidSdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/invalid.sdp').readAsString();
+      await new File('./test/sdp_test_data/invalid.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -354,7 +354,7 @@ _testInvalidSdp() async {
 }
 
 _testJssipSdp() async {
-  String sdp = await new File('./tests/sdp_test_data/jssip.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/jssip.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -410,7 +410,7 @@ _testJssipSdp() async {
 }
 
 _testJsepSdp() async {
-  String sdp = await new File('./tests/sdp_test_data/jsep.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/jsep.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -441,7 +441,7 @@ _testJsepSdp() async {
 }
 
 _testAlacSdp() async {
-  String sdp = await new File('./tests/sdp_test_data/alac.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/alac.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -461,7 +461,7 @@ _testAlacSdp() async {
 }
 
 testOnvifSdp() async {
-  String sdp = await new File('./tests/sdp_test_data/onvif.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/onvif.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -500,7 +500,7 @@ testOnvifSdp() async {
 }
 
 _testSsrcSdp() async {
-  String sdp = await new File('./tests/sdp_test_data/ssrc.sdp').readAsString();
+  String sdp = await new File('./test/sdp_test_data/ssrc.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -519,7 +519,7 @@ _testSsrcSdp() async {
 
 _testSimulcastSdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/simulcast.sdp').readAsString();
+      await new File('./test/sdp_test_data/simulcast.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -716,7 +716,7 @@ _testSimulcastSdp() async {
 
 _testSt2202_6Sdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/st2022-6.sdp').readAsString();
+      await new File('./test/sdp_test_data/st2022-6.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -736,7 +736,7 @@ _testSt2202_6Sdp() async {
 
 _testSt2110_20Sdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/st2110-20.sdp').readAsString();
+      await new File('./test/sdp_test_data/st2110-20.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -774,7 +774,7 @@ _testSt2110_20Sdp() async {
 
 _testSctpDtls26Sdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/sctp-dtls-26.sdp').readAsString();
+      await new File('./test/sdp_test_data/sctp-dtls-26.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -806,7 +806,7 @@ _testSctpDtls26Sdp() async {
 
 _testExtmapEncryptSdp() async {
   String sdp =
-      await new File('./tests/sdp_test_data/extmap-encrypt.sdp').readAsString();
+      await new File('./test/sdp_test_data/extmap-encrypt.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -864,7 +864,7 @@ _testExtmapEncryptSdp() async {
 
 _testDanteAes67() async {
   String sdp =
-      await new File('./tests/sdp_test_data/dante-aes67.sdp').readAsString();
+      await new File('./test/sdp_test_data/dante-aes67.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -900,7 +900,7 @@ _testDanteAes67() async {
 
 _testTcpActive() async {
   String sdp =
-      await new File('./tests/sdp_test_data/tcp-active.sdp').readAsString();
+      await new File('./test/sdp_test_data/tcp-active.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');
@@ -927,7 +927,7 @@ _testTcpActive() async {
 
 _testTcpPassive() async {
   String sdp =
-      await new File('./tests/sdp_test_data/tcp-passive.sdp').readAsString();
+      await new File('./test/sdp_test_data/tcp-passive.sdp').readAsString();
 
   Map<String, dynamic> session = parse(sdp + '');
   ok(session, 'got session info');