Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/stacktrace_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 '../source_map_validator_helper.dart';
 
 const String EXCEPTION_MARKER = '>ExceptionMarker<';
@@ skipping 531 matching lines @@
   const LineException('_asyncStart.<anonymous function>', 'async_patch.dart'),
   const LineException('_RootZone.runUnary', 'zone.dart'),
   const LineException('_FutureListener.handleValue', 'future_impl.dart'),
   const LineException('_Future._completeWithValue', 'future_impl.dart'),
   const LineException(
       '_Future._propagateToListeners.handleValueCallback', 'future_impl.dart'),
   const LineException('_Future._propagateToListeners', 'future_impl.dart'),
   const LineException(
       '_Future._addListener.<anonymous function>', 'future_impl.dart'),
 ];