Improve incremental compilation.

dart/tests/compiler/dart2js/incremental/incremental_helper.dart

Index: dart/tests/compiler/dart2js/incremental/incremental_helper.dart
diff --git a/dart/pkg/docgen/lib/src/exports/dart2js_mirrors.dart b/dart/tests/compiler/dart2js/incremental/incremental_helper.dart
similarity index 56%
copy from dart/pkg/docgen/lib/src/exports/dart2js_mirrors.dart
copy to dart/tests/compiler/dart2js/incremental/incremental_helper.dart
index a380c3810bd0326fe393f9ef16e4f8f51f042f0a..3bdf8e3805871396bb29a913d3efa7ca5b4153f1 100644
--- a/dart/pkg/docgen/lib/src/exports/dart2js_mirrors.dart
+++ b/dart/tests/compiler/dart2js/incremental/incremental_helper.dart
@@ -2,6 +2,9 @@
 // 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 docgen.exports.dart2js_mirrors;
-
-export 'package:compiler/implementation/mirrors/dart2js_mirrors.dart';
+const List<String> INCREMENTAL_OPTIONS = const <String>[
+    '--analyze-main',
+    '--disable-type-inference',
+    '--incremental-support',
+    '--no-source-maps',
+];