Enable and fix lint unnecessary_this (#314)

Author: natebosch

analysis_options.yaml

@@ -72,7 +72,7 @@ linter:
   - unnecessary_overrides
   - unnecessary_parenthesis
   - unnecessary_statements
-  # unnecessary_this
+  - unnecessary_this
   - unrelated_type_equality_checks
   - use_rethrow_when_possible
   - valid_regexps

lib/src/multipart_file.dart

@@ -45,9 +45,8 @@ class MultipartFile {
   /// future may be inferred from [filename].
   MultipartFile(this.field, Stream<List<int>> stream, this.length,
       {this.filename, MediaType contentType})
-      : this._stream = toByteStream(stream),
-        this.contentType =
-            contentType ?? MediaType('application', 'octet-stream');
+      : _stream = toByteStream(stream),
+        contentType = contentType ?? MediaType('application', 'octet-stream');
 
   /// Creates a new [MultipartFile] from a byte array.
   ///

lib/src/request.dart

@@ -127,7 +127,7 @@ class Request extends BaseRequest {
           'content-type "${contentType.mimeType}".');
     }
 
-    this.body = mapToQuery(fields, encoding: encoding);
+    body = mapToQuery(fields, encoding: encoding);
   }
 
   /// Creates a new HTTP request.

lib/src/streamed_response.dart

@@ -25,7 +25,7 @@ class StreamedResponse extends BaseResponse {
       bool isRedirect = false,
       bool persistentConnection = true,
       String reasonPhrase})
-      : this.stream = toByteStream(stream),
+      : stream = toByteStream(stream),
         super(statusCode,
             contentLength: contentLength,
             request: request,