All strong mode cleaning of dart2js.

tests/compiler/dart2js/sourcemaps/mapping_test.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.
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/compiler_new.dart';
-import 'package:compiler/src/apiimpl.dart';
 import 'package:compiler/src/commandline_options.dart';
-import 'package:compiler/src/dart2js.dart' as entry;
 import 'package:expect/expect.dart';
 import 'package:source_maps/source_maps.dart';
-import 'package:source_maps/src/utils.dart';
 
 import '../annotated_code_helper.dart';
 import '../memory_compiler.dart';
-import '../source_map_validator_helper.dart';
 
 const List<String> TESTS = const <String>[
   '''
 @{main}main() {
 @{main}}
 ''',
   '''
 @{main}main() {
   @{+main}throw '';
 @{main}}
@@ skipping 177 matching lines @@
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! SourceLocation) return false;
     return methodName == other.methodName &&
         lineNo == other.lineNo &&
         columnNo == other.columnNo;
   }
 
   String toString() => '$methodName:$lineNo:$columnNo';
 }