First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/analyze_only_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.
 
 // Smoke test of the dart2js compiler API.
 library analyze_only;
 
 import "package:expect/expect.dart";
 import 'dart:async';
+import "package:async_helper/async_helper.dart";
 
 import '../../utils/dummy_compiler_test.dart' as dummy;
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 
 runCompiler(String main, List<String> options,
             onValue(String code, List errors, List warnings)) {
   List errors = new List();
   List warnings = new List();
 
   Future<String> localProvider(Uri uri) {
     if (uri.scheme != 'main') return dummy.provider(uri);
     return new Future<String>.value(main);
   }
 
   void localHandler(Uri uri, int begin, int end,
                     String message, Diagnostic kind) {
     dummy.handler(uri, begin, end, message, kind);
     if (kind == Diagnostic.ERROR) {
       errors.add(message);
     } else if (kind == Diagnostic.WARNING) {
       warnings.add(message);
     }
   }
 
   print('-----------------------------------------------');
   print('main source:\n$main');
   print('options: $options\n');
+  asyncStart();
   Future<String> result =
       compile(new Uri(scheme: 'main'),
               new Uri(scheme: 'lib', path: '/'),
               new Uri(scheme: 'package', path: '/'),
               localProvider, localHandler, options);
   result.then((String code) {
     onValue(code, errors, warnings);
   }, onError: (e) {
       throw 'Compilation failed';
-  });
+  }).whenComplete(() => asyncEnd());
 }
 
 main() {
   runCompiler(
     "",
     [],
     (String code, List errors, List warnings) {
       Expect.isNull(code);
       Expect.equals(1, errors.length);
       Expect.equals('Error: Could not find "main".', errors[0].toString());
@@ skipping 98 matching lines @@
   // --analyze-signatures-only implies --analyze-only
   runCompiler(
     "",
     ['--analyze-signatures-only', '--analyze-all'],
     (String code, List errors, List warnings) {
       Expect.isNull(code);
       Expect.isTrue(errors.isEmpty);
       Expect.isTrue(warnings.isEmpty);
     });
 }