All strong mode cleaning of dart2js.

tests/compiler/dart2js/sourcemaps/nomapping_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() {}
 ''',
 ];
 
 void main(List<String> arguments) {
   bool verbose = false;
   bool writeJs = false;
@@ skipping 90 matching lines @@
   }
   if (entry.sourceLine != null) {
     sb.write('sourceLine=');
     sb.write(entry.sourceLine);
     sb.write(',sourceColumn=');
     sb.write(entry.sourceColumn);
   }
   sb.write(']');
   return sb.toString();
 }