Remove warnings from dart2js test-suite.

tests/compiler/dart2js/source_mapping_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import "../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import "mock_compiler.dart";
-import 'parser_helper.dart';
+import '../../../sdk/lib/_internal/compiler/implementation/js_backend/js_backend.dart';
 
 Future<CodeBuffer> compileAll(SourceFile sourceFile) {
   MockCompiler compiler = new MockCompiler();
   Uri uri = new Uri(path: sourceFile.filename);
   compiler.sourceFiles[uri.toString()] = sourceFile;
+  JavaScriptBackend backend = compiler.backend;
   return compiler.runCompiler(uri).then((_) {
-    return compiler.backend.emitter.mainBuffer;
+    return backend.emitter.mainBuffer;
   });
 }
 
 void testSourceMapLocations(String codeWithMarkers) {
   List<int> expectedLocations = new List<int>();
   for (int i = 0; i < codeWithMarkers.length; ++i) {
     if (codeWithMarkers[i] == '@') {
       expectedLocations.add(i - expectedLocations.length);
     }
   }
@@ skipping 74 matching lines @@
   testSourceMapLocations(FUNCTIONS_TEST);
   testSourceMapLocations(RETURN_TEST);
   testSourceMapLocations(NOT_TEST);
   testSourceMapLocations(UNARY_TEST);
   testSourceMapLocations(BINARY_TEST);
   testSourceMapLocations(SEND_TEST);
   testSourceMapLocations(SEND_SET_TEST);
   testSourceMapLocations(LOOP_TEST);
   testSourceMapLocations(INTERCEPTOR_TEST);
 }