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

tests/compiler/dart2js/js_backend_cps_ir_interceptors_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 interceptors.
 
 library interceptors_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main() {
   var g = 1;
 
   var x = g + 3;
   print(x);
 }""",
 r"""
 function() {
   P.print(4);
-  return null;
 }"""),
   const TestEntry("""
 main() {
   var l = ['hest', ['h', 'e', 's', 't']];
   print(l.length);
   for (int i  = 0; i < l.length; i++) {
     var x = l[i];
     for (int j = 0; j < x.length; j++) {
       print(x[j]);
     }
   }
 }""",
 r"""
 function() {
   var l = ["hest", ["h", "e", "s", "t"]], i = 0, x, j;
   P.print(J.getInterceptor$as(l).get$length(l));
   while (i < J.getInterceptor$as(l).get$length(l)) {
     x = J.getInterceptor$as(l).$index(l, i);
     j = 0;
     while (j < J.getInterceptor$as(x).get$length(x)) {
       P.print(J.getInterceptor$as(x).$index(x, j));
       j = j + 1;
     }
     i = i + 1;
   }
-  return null;
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }