First pass at asynchronous input loading in dart2js.

tests/utils/recursive_import_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.
 
 // Test of "recursive" imports using the dart2js compiler API.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
@@ skipping 51 matching lines @@
 main() {}
 """;
 
 
 main() {
   int count = 0;
   Future<String> provider(Uri uri) {
     String source;
     if (uri.path.length > 100) {
       // Simulate an OS error.
-      throw 'Path length exceeded';
+      return new Future.error('Path length exceeded');
     } else if (uri.scheme == "main") {
       count++;
       source = RECURSIVE_MAIN;
     } else if (uri.scheme == "lib") {
       if (uri.path.endsWith("/core.dart")) {
         source = CORE_LIB;
       } else if (uri.path.endsWith('_patch.dart')) {
         source = '';
       } else if (uri.path.endsWith('isolate_helper.dart')) {
         source = 'class _WorkerStub {}';
       } else if (uri.path.endsWith('interceptors.dart')) {
         source = INTERCEPTORS_LIB;
       } else {
         source = "library lib${uri.path.replaceAll('/', '.')};";
       }
     } else {
-     throw "unexpected URI $uri";
+     return new Future.error("unexpected URI $uri");
     }
     return new Future.value(source);
   }
 
   int warningCount = 0;
   int errorCount = 0;
   void handler(Uri uri, int begin, int end, String message, Diagnostic kind) {
     if (uri != null) {
       // print('$uri:$begin:$end: $kind: $message');
       Expect.equals('main', uri.scheme);
@@ skipping 16 matching lines @@
     Expect.isNull(code);
     Expect.isTrue(10 < count);
     // Two warnings for each time RECURSIVE_MAIN is read, except the
     // first time.
     Expect.equals(2 * (count - 1), warningCount);
     Expect.equals(1, errorCount);
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }