Add end-to-end test for consistency in source map file and js file.

tests/compiler/dart2js/source_map_consistency_helper.dart

+// Copyright (c) 2014, 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:io';
+import 'dart:convert';
+import 'dart:async';
+
+import 'package:path/path.dart' as path;
+import 'package:expect/expect.dart';
+import 'package:source_maps/source_maps.dart';
+
+checkConsistency(Uri outUri) {
+  print('Accessing $outUri');
+  File sourceFile = new File.fromUri(outUri);
+  Expect.isTrue(sourceFile.existsSync());
+  String mapName = getMapReferenceFromJsOutput(sourceFile.readAsStringSync());
+  Uri mapUri = outUri.resolve(mapName);
+  print('Accessing $mapUri');
+  File mapFile = new File.fromUri(mapUri);
+  Expect.isTrue(mapFile.existsSync());
+  SingleMapping sourceMap = new SingleMapping.fromJson(
+      JSON.decode(mapFile.readAsStringSync()));
+  Expect.equals(outUri, mapUri.resolve(sourceMap.targetUrl));
+}
+
+String getMapReferenceFromJsOutput(String file) {
+  List<String> out = file.split('\n');
+  String mapReference = out[out.length - 3]; // #sourceMappingURL=<url>
+  Expect.isTrue(mapReference.startsWith('//# sourceMappingURL='));
+  return mapReference.substring(mapReference.indexOf('=') + 1);
+}
+
+copyDirectory(Directory sourceDir, Directory destinationDir) {
+  sourceDir.listSync().forEach((FileSystemEntity element) {
+    String newPath = path.join(destinationDir.path,
+                               path.basename(element.path));
+    if (element is File) {
+      element.copySync(newPath);
+    } else if (element is Directory) {
+      Directory newDestinationDir = new Directory(newPath);
+      newDestinationDir.createSync();
+      copyDirectory(element, newDestinationDir);
+    }
+  });
+}
+
+Future<Directory> createTempDir() {
+  return Directory.systemTemp
+      .createTemp('sourceMap_test-')
+      .then((Directory dir) {
+    return dir;
+  });
+}