Move js_ast library to its own package under pkg

pkg/compiler/lib/src/js/js_ast.dart

Index: pkg/compiler/lib/src/js/js_ast.dart
diff --git a/pkg/compiler/lib/src/js/js_ast.dart b/pkg/compiler/lib/src/js/js_ast.dart
deleted file mode 100644
index ee8289ecf2df0aa8bf0c73727d8dac478d223ff5..0000000000000000000000000000000000000000
--- a/pkg/compiler/lib/src/js/js_ast.dart
+++ /dev/null
@@ -1,13 +0,0 @@
-// 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.
-
-library js_ast;
-
-import 'precedence.dart';
-import 'characters.dart' as charCodes;
-
-part 'nodes.dart';
-part 'builder.dart';
-part 'printer.dart';
-part 'template.dart';