Enable and fix lint prefer_typing_uninitialized_variables (#312)

Author: natebosch

analysis_options.yaml

@@ -55,7 +55,7 @@ linter:
   - prefer_is_not_empty
   - prefer_null_aware_operators
   - prefer_single_quotes
-  # prefer_typing_uninitialized_variables
+  - prefer_typing_uninitialized_variables
   - recursive_getters
   - slash_for_doc_comments
   - test_types_in_equals

test/io/multipart_test.dart

@@ -14,7 +14,7 @@ import 'package:test/test.dart';
 import 'utils.dart';
 
 void main() {
-  var tempDir;
+  Directory tempDir;
   setUp(() {
     tempDir = Directory.systemTemp.createTempSync('http_test_');
   });

test/io/utils.dart

@@ -73,7 +73,7 @@ Future startServer() {
             ContentType('application', 'json', charset: outputEncoding.name);
         response.headers.set('single', 'value');
 
-        var requestBody;
+        dynamic requestBody;
         if (requestBodyBytes.isEmpty) {
           requestBody = null;
         } else if (request.headers.contentType?.charset != null) {

test/utils.dart

@@ -51,7 +51,7 @@ class _Parse extends Matcher {
   bool matches(item, Map matchState) {
     if (item is! String) return false;
 
-    var parsed;
+    dynamic parsed;
     try {
       parsed = json.decode(item);
     } catch (e) {