Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/save.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.
 
 /// Loads a human-readable source map and outputs the source map file for it.
 
 library save;
 
 import 'dart:convert';
 import 'dart:io';
@@ skipping 26 matching lines @@
 
   if (sourceMapFile != null) {
     sourceMapFile.writeAsStringSync(JSON.encoder.convert(mapping.toJson()));
   } else {
     print(new JsonEncoder.withIndent('  ').convert(mapping.toJson()));
   }
 }
 
 SingleMapping convertFromHumanReadableSourceMap(String json) {
   Map inputMap = lazon.decode(json);
-  String file = inputMap['file'];
   Map urls = inputMap['sources'];
   List<String> sources = new List<String>.filled(urls.length, null);
   urls.forEach((String index, String url) {
     int i = int.parse(index);
     assert(sources[i] == null);
     sources[i] = url;
   });
   List lines = inputMap['lines'];
   Map<String, int> names = <String, int>{};
   Map<int, TargetLineEntry> lineEntryMap = {};
@@ skipping 74 matching lines @@
     'file': inputMap['file'],
     'sources': sources,
     'names': names.keys.toList(),
     'mappings': '',
   };
 
   SingleMapping mapping = new SingleMapping.fromJson(outputMap);
   mapping.lines.addAll(lineEntries);
   return mapping;
 }