Improve incremental compilation.

dart/site/try/src/compiler_isolate.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 compiler_isolate;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:isolate';
 import 'dart:convert' show JSON;
@@ skipping 91 matching lines @@
       throw new Exception('Throw on error');
     }
   }
   Stopwatch compilationTimer = new Stopwatch()..start();
   cachedCompiler = reuseCompiler(
       diagnosticHandler: handler,
       inputProvider: inputProvider,
       options: options,
       cachedCompiler: cachedCompiler,
       libraryRoot: sdkLocation,
-      packageRoot: Uri.base.resolve('/packages/'));
+      packageRoot: Uri.base.resolve('/packages/'),
+      packagesAreImmutable: true);
 
   cachedCompiler.run(Uri.parse('$PRIVATE_SCHEME:/main.dart')).then((success) {
     compilationTimer.stop();
     print('Compilation took ${compilationTimer.elapsed}');
     if (cachedCompiler.libraries.containsKey('dart:html')) {
       notifyDartHtml(replyTo);
     }
     String js = cachedCompiler.assembledCode;
     if (js == null) {
       if (!options.contains('--analyze-only')) replyTo.send('failed');
@@ skipping 29 matching lines @@
   port.send(replyTo.sendPort);
   replyTo.listen((message) {
     try {
       List list = message as List;
       compile(list[0], list[1]);
     } catch (exception, stack) {
       port.send('$exception\n$stack');
     }
   });
 }