Add CompilationUnit abstraction for tracking "project files".

dart/site/try/src/leap.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.
 
 library trydart.main;
 
 import 'dart:html' show
     HttpRequest,
     LinkElement,
     querySelector,
     window;
 
 import 'dart:isolate' show
     ReceivePort,
     SendPort;
 
 import 'compilation.dart' show
     compilerIsolate,
-    compilerPort;
+    compilerPort,
+    currentSource;
 
 import 'isolate_legacy.dart' show
     spawnDomFunction,
     spawnFunction;
 
 import 'samples.dart' show
     EXAMPLE_HELLO;
 
 import 'ui.dart' show
     buildUI,
@@ skipping 13 matching lines @@
   try {
     new HttpRequest();
     replyTo.send(HAS_NON_DOM_HTTP_REQUEST);
   } catch (e, trace) {
     replyTo.send(NO_NON_DOM_HTTP_REQUEST);
   }
 }
 
 main() {
   UserOption.storage = window.localStorage;
-  if (window.localStorage['currentSource'] == null) {
-    window.localStorage['currentSource'] = EXAMPLE_HELLO;
+  if (currentSource == null) {
+    currentSource = EXAMPLE_HELLO;
   }
 
   buildUI();
   spawnFunction(checkHttpRequest).first.then((reply) {
     ReceivePort port;
     if (reply == HAS_NON_DOM_HTTP_REQUEST) {
       port = spawnFunction(compilerIsolate);
     } else {
       port = spawnDomFunction(compilerIsolate);
     }
@@ skipping 15 matching lines @@
           interaction.onMutation([], observer);
           break;
         default:
           // TODO(ahe): Close [port]?
           print('Unexpected message received: $message');
           break;
       }
     });
   });
 }