Support for map literals in cps js emitter

tests/compiler/dart2js/js_backend_cps_ir_literals.dart

 // Copyright (c) 2014, 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.
 
 // Tests of literals.
 
 library literals_tests;
 
 import 'js_backend_cps_ir_test.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main() {
   print([]);
   print([1]);
   print([1, 2]);
   print([1, [1, 2]]);
+  print({});
+  print({1: 2});
+  print({[1, 2]: [3, 4]});
 }
 """,
-"""
+r"""
 function() {
   P.print([]);
   P.print([1]);
   P.print([1, 2]);
   P.print([1, [1, 2]]);
+  P.print(P.LinkedHashMap_LinkedHashMap$_empty());
+  P.print(P.LinkedHashMap_LinkedHashMap$_literal([1, 2]));
+  P.print(P.LinkedHashMap_LinkedHashMap$_literal([[1, 2], [3, 4]]));
   return null;
 }"""),
 ];