dart2js cps: Better fallthrough analysis and eliminate "return null".

tests/compiler/dart2js/js_backend_cps_ir_literals_test.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.dart';
 
@@ skipping 11 matching lines @@
 """,
 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;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }