Add interfaces for a new compiler API.

tests/compiler/dart2js/memory_source_file_helper.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.test.memory_source_file_helper;
 
 import 'dart:async' show Future;
 export 'dart:io' show Platform;
 
 export 'package:compiler/src/apiimpl.dart'
@@ skipping 34 matching lines @@
   SourceFile getSourceFile(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.getSourceFile(resourceUri);
     }
     String source = memorySourceFiles[resourceUri.path];
     if (source == null) {
       throw new Exception('No such file $resourceUri');
     }
     return new StringSourceFile.fromUri(resourceUri, source);
   }
+
+  @override
+  Future readFromUri(Uri resourceUri) => readStringFromUri(resourceUri);
 }