First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/package_root_test.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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.package_root;
 
 import 'package:expect/expect.dart';
 import 'memory_source_file_helper.dart';
@@ skipping 41 matching lines @@
     return new NullSink('$name.$extension');
   }
 
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
                                    null,
                                    []);
 
-  compiler.run(main);
-  Expect.equals(1, errors.length);
-  Expect.equals('Error: Cannot resolve "package:foo/foo.dart". '
-                'Package root has not been set.',
-                errors[0]);
+  compiler.run(main).then((_) {
+    Expect.equals(1, errors.length);
+    Expect.equals('Error: Cannot resolve "package:foo/foo.dart". '
+                  'Package root has not been set.',
+                  errors[0]);
+  });
 }
 
 void main() {
   runCompiler(Uri.parse('memory:main.dart'));
   runCompiler(Uri.parse('package:foo/foo.dart'));
 }