Add LibraryLoader.resetAsync.

dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart

 // Copyright (c) 2014, 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_incremental;
 
 import 'dart:async' show
     Future;
 
 import 'dart:profiler' show
@@ skipping 51 matching lines @@
       throw new ArgumentError('inputProvider is null.');
     }
     if (diagnosticHandler == null) {
       throw new ArgumentError('diagnosticHandler is null.');
     }
   }
 
   Future<bool> compile(Uri script) {
     List<String> options = new List<String>.from(this.options);
     options.addAll(INCREMENTAL_OPTIONS);
-    _compiler = reuseCompiler(
+    Future<Compiler> future = reuseCompiler(
         cachedCompiler: _compiler,
         libraryRoot: libraryRoot,
         packageRoot: packageRoot,
         inputProvider: inputProvider,
         diagnosticHandler: diagnosticHandler,
         options: options,
         outputProvider: outputProvider,
         environment: environment);
-    return _compiler.run(script);
+    return future.then((Compiler compiler) {
+      _compiler = compiler;
+      return compiler.run(script);
+    });
   }
 }