First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/codegen_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.
 
+import 'dart:async';
 import 'package:expect/expect.dart';
 
 import 'memory_source_file_helper.dart';
 import "../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart"
     show SourceString;
 
-Map<String, String> generate(String code, [List<String> options = const []]) {
+Future<Map<String, String>> generate(String code,
+    [List<String> options = const []]) {
   Uri script = currentDirectory.resolve(nativeToUriPath(new Options().script));
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   MemorySourceFileProvider.MEMORY_SOURCE_FILES = { 'main.dart': code };
   var provider = new MemorySourceFileProvider();
   var handler = new FormattingDiagnosticHandler(provider);
 
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    null,
                                    handler.diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    options);
   Uri uri = Uri.parse('memory:main.dart');
-  Expect.isTrue(compiler.run(uri));
-  Map<String, String> result = new Map<String, String>();
-  for (var element in compiler.backend.generatedCode.keys) {
-    if (element.getCompilationUnit().script.uri != uri) continue;
-    var name = element.name.slowToString();
-    var code = compiler.backend.assembleCode(element);
-    result[name] = code;
-  }
-  return result;
+  return compiler.run(uri).then((success) {
+    Expect.isTrue(success);
+    Map<String, String> result = new Map<String, String>();
+    for (var element in compiler.backend.generatedCode.keys) {
+      if (element.getCompilationUnit().script.uri != uri) continue;
+      var name = element.name.slowToString();
+      var code = compiler.backend.assembleCode(element);
+      result[name] = code;
+    }
+    return result;
+  });
 }