Fix const unparse. Example:

tests/compiler/dart2js/unparser_test.dart

 // Copyright (c) 2012, 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.
 
 #import('dart:uri');
 #import('parser_helper.dart');
 #import("../../../lib/compiler/compiler.dart");
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 
 testUnparse(String statement) {
@@ skipping 76 matching lines @@
   testUnparseMember('foo()native "bar";');
   testUnparseMember('foo()native "this.x = 41";');
 }
 
 testPrefixIncrements() {
   testUnparse('++i;');
   testUnparse('++a[i];');
   testUnparse('++a[++b[i]];');
 }
 
+testConstModifier() {
+  testUnparse('foo([var a=const []]){}');
+  testUnparse('foo([var a=const{}]){}');
+  testUnparse('foo(){var a=const []; var b=const{};}');
+  testUnparse('foo([var a=const [const{"a": const [1, 2, 3]}]]){}');
+}
+
 testSimpleFileUnparse() {
   final src = '''
 should_be_dropped() {
 }
 
 should_be_kept() {
 }
 
 main() {
   should_be_kept();
 }
 ''';
   testDart2Dart(src, (String s) {
     Expect.equals('should_be_kept(){}main(){should_be_kept();}', s);
   });
 }
 
 main() {
   testGenericTypes();
   testForLoop();
   testEmptyList();
   testClosure();
   testIndexedOperatorDecl();
   testNativeMethods();
   testPrefixIncrements();
+  testConstModifier();
   testSimpleFileUnparse();
 }