Refactor Future constructors.

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;
 import 'dart:uri' show Uri;
 export 'dart:uri' show Uri;
 import 'dart:io';
@@ skipping 14 matching lines @@
 export '../../../sdk/lib/_internal/compiler/implementation/source_file_provider.dart'
        show FormattingDiagnosticHandler;
 
 class MemorySourceFileProvider extends SourceFileProvider {
   static Map MEMORY_SOURCE_FILES;
   Future<String> readStringFromUri(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.readStringFromUri(resourceUri);
     }
     String source = MEMORY_SOURCE_FILES[resourceUri.path];
-    // TODO(ahe): Return new Future.immediateError(...) ?
+    // TODO(ahe): Return new Future.error(...) ?
     if (source == null) throw 'No such file $resourceUri';
     String resourceName = '$resourceUri';
     this.sourceFiles[resourceName] = new SourceFile(resourceName, source);
-    return new Future.immediate(source);
+    return new Future.value(source);
   }
 }