Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/source_mapping_test_viewer.dart

 // Copyright (c) 2015, 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.
 
 /// Visualization of source mappings generated and tested in
 /// 'source_mapping_test.dart'.
 
 library source_mapping.test.viewer;
 
 import 'dart:async';
 import 'package:compiler/src/filenames.dart';
 import 'package:compiler/src/util/util.dart';
 import 'source_mapping_tester.dart';
-import 'sourcemap_helper.dart';
 import 'sourcemap_html_helper.dart';
 import 'sourcemap_html_templates.dart';
 
 const String DEFAULT_OUTPUT_PATH = 'out.js.map.html';
 
 main(List<String> arguments) async {
   bool measure = false;
   String outputPath = DEFAULT_OUTPUT_PATH;
   Set<String> configurations = new Set<String>();
   Set<String> files = new Set<String>();
@@ skipping 124 matching lines @@
   final int count;
 
   Measurement(this.config, this.filename, this.missing, this.count);
 
   String toString() {
     double percentage = 100 * missing / count;
     return "Config '${config}', file: '${filename}': "
         "$missing of $count ($percentage%) missing";
   }
 }