fixes #131, use before define from variables to classes

lib/src/dependency_graph.dart

 // Copyright (c) 2015, 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.
 
 /// Tracks the shape of the import/export graph and dependencies between files.
 library dev_compiler.src.dependency_graph;
 
 import 'dart:collection' show HashSet;
 
 import 'package:analyzer/analyzer.dart' show parseDirectives;
@@ skipping 457 matching lines @@
   return files;
 }();
 
 /// Curated order to minimize lazy classes needed by dart:core and its
 /// transitive SDK imports.
 const corelibOrder = const [
   'dart.core',
   'dart.collection',
   'dart._internal',
   'dart.math',
+  'dart.async',

[Jennifer Messerly, 2015/05/12 16:23:46]

stole this from your CL. Gets sunflower working

   'dart._foreign_helper',
+  'dart._js_embedded_names',
+  'dart._js_names',
   'dart._js_helper',
+  'dart.isolate',
+  'dart.typed_data',
+  'dart._isolate_helper',
+  'dart._js_primitives',
+
   // TODO(jmesserly): add others
   /*
-  'dart.async',
   'dart.convert',
-  'dart.isolate',
-  'dart.typed_data',
   'dart._foreign_helper',
   'dart._interceptors',
-  'dart._isolate_helper',
-  'dart._js_embedded_names',
-  'dart._js_names',
-  'dart._js_primitives',
   'dart._native_typed_data',
   */
 ];
 
 /// Runtime files added to applications when running in server mode.
 final runtimeFilesForServerMode = new List<String>.from(defaultRuntimeFiles)
   ..addAll(const ['messages_widget.js', 'messages.css']);
 
 final _log = new Logger('dev_compiler.dependency_graph');