All strong mode cleaning of dart2js.

tests/compiler/dart2js/sourcemaps/diff_view.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library sourcemap.diff_view;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 127 matching lines @@
 
   @override
   decodeLineAnnotation(json) {
     if (json != null) {
       return CodeSource.fromJson(json);
     }
     return null;
   }
 
   @override
-  encodeLineAnnotation(CodeSource lineAnnotation) {
+  encodeLineAnnotation(covariant CodeSource lineAnnotation) {
     if (lineAnnotation != null) {
       return lineAnnotation.toJson();
     }
     return null;
   }
 }
 
 /// The structured output of a compilation.
 class AnnotatedOutput {
   final String filename;
@@ skipping 795 matching lines @@
     if (data.annotationType == AnnotationType.WITHOUT_SOURCE_INFO) {
       return new AnnotationData(tag: 'span', properties: {
         'title': annotation.title,
         'class': '${ClassNames.marker} '
             '${data.annotationType.className}'
       });
     }
   }
   return null;
 }