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

tests/compiler/dart2js/js_backend_cps_ir_operators_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 operators.
 
 library operators_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("main() { return true ? 42 : 'foo'; }"),
   const TestEntry("""
 foo() => foo();
 main() {
   print(foo() ? "hello world" : "bad bad");
 }""",
 """function() {
   V.foo();
   P.print("bad bad");
-  return null;
 }"""),
   const TestEntry("""
 foo() => null;
 main() {
   print(foo() ? "hello world" : "bad bad");
 }""",
 """function() {
   V.foo();
   P.print("bad bad");
-  return null;
 }"""),
   const TestEntry("""
 get foo => foo;
 main() {
   print(foo ? "hello world" : "bad bad");
 }""",
 """function() {
   V.foo();
   P.print("bad bad");
-  return null;
 }"""),
   const TestEntry("""
 get foo => foo;
 main() { print(foo && foo); }""",
 """function() {
   V.foo();
   P.print(false);
-  return null;
 }"""),
   const TestEntry("""
 get foo => foo;
 main() { print(foo || foo); }""",
 """function() {
   V.foo();
   V.foo();
   P.print(false);
-  return null;
 }"""),
 
 // Needs interceptor calling convention
 //const TestEntry("""
 //class Foo {
 //  operator[]=(index, value) {
 //    print(value);
 //  }
 //}
 //main() {
 //  var foo = new Foo();
 //  foo[5] = 6;
 //}""", r"""
 //function() {
 //  V.Foo$().$indexSet(5, 6);
 //}
 //"""),
 
 const TestEntry("""
 main() {
   var list = [1, 2, 3];
   list[1] = 6;
   print(list);
 }""", r"""
 function() {
   var list = [1, 2, 3];
   J.getInterceptor$a(list).$indexSet(list, 1, 6);
   P.print(list);
-  return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }