Strong mode cleaning of many dart2js tests.

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';
 
-import 'package:compiler/src/common.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/diagnostics/invariant.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/io/position_information.dart';
 import 'package:compiler/src/io/source_information.dart';
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/js/js.dart' as js;
-import 'package:compiler/src/js/js_debug.dart';
 
 import 'diff.dart';
 import 'html_parts.dart';
 import 'js_tracer.dart';
 import 'output_structure.dart';
 import 'sourcemap_helper.dart';
 import 'sourcemap_html_helper.dart';
 import 'trace_graph.dart';
 
 main(List<String> args) async {
@@ skipping 926 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;
 }