First pass at asynchronous input loading in dart2js.

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:io';
 export 'dart:io' show Platform;
 
@@ skipping 13 matching lines @@
        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.error(...) ?
-    if (source == null) throw 'No such file $resourceUri';
+    if (source == null) return new Future.error('No such file $resourceUri');
     String resourceName = '$resourceUri';
     this.sourceFiles[resourceName] = new SourceFile(resourceName, source);
     return new Future.value(source);
   }
 }