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

tests/compiler/dart2js/js_backend_cps_ir_closures_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 closures.
 
 library closures_test;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main(x) {
   a() {
     return x;
   }
   x = x + '1';
   print(a());
 }
 """,
 r"""
 function(x) {
   var _box_0 = {}, a = new V.main_a(_box_0);
   _box_0._captured_x_0 = x;
   _box_0._captured_x_0 = J.getInterceptor$ns(x = _box_0._captured_x_0).$add(x, "1");
   P.print(a.call$0());
-  return null;
 }"""),
 
   const TestEntry("""
 main(x) {
   a() {
     return x;
   }
   print(a());
 }
 """,
 r"""
 function(x) {
   P.print(new V.main_a(x).call$0());
-  return null;
 }"""),
 
   const TestEntry("""
 main() {
   var x = 122;
   var a = () => x;
   x = x + 1;
   print(a());
 }
 """,
 r"""
 function() {
   var _box_0 = {}, a = new V.main_closure(_box_0);
   _box_0._captured_x_0 = 122;
   _box_0._captured_x_0 = _box_0._captured_x_0 + 1;
   P.print(a.call$0());
-  return null;
 }"""),
 
   const TestEntry("""
 main() {
   var x = 122;
   var a = () {
     var y = x;
     return () => y;
   };
   x = x + 1;
   print(a()());
 }
 """,
 r"""
 function() {
   var _box_0 = {}, a = new V.main_closure(_box_0);
   _box_0._captured_x_0 = 122;
   _box_0._captured_x_0 = _box_0._captured_x_0 + 1;
   P.print(a.call$0().call$0());
-  return null;
 }"""),
 
   const TestEntry("""
 main() {
   var a;
   for (var i=0; i<10; i++) {
     a = () => i;
   }
   print(a());
 }
 """,
 r"""
 function() {
   var a = null, i = 0;
   while (i < 10) {
     a = new V.main_closure(i);
     i = i + 1;
   }
   P.print(a.call$0());
-  return null;
 }"""),
 
   const TestEntry.forMethod('function(A#b)', """
 class A {
   a() => 1;
   b() => () => a();
 }
 main() {
   print(new A().b()());
 }
 """,
 r"""
 function() {
   return new V.A_b_closure(this);
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }