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

tests/compiler/dart2js/js_backend_cps_ir_constructor_test.dart

 // Copyright (c) 2015, 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 constructor_test;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 25 matching lines @@
   Sub(x, this.y) : super(x) {
     print(x);
   }
 }
 main() {
   print(new Sub(1, 2).x);
 }""",
 r"""
 function(x, y) {
   P.print(x);
-  return null;
 }"""),
 
   const TestEntry.forMethod('generative_constructor(Sub#)', """
 class Base0 {
   Base0() {
     print('Base0');
   }
 }
 class Base extends Base0 {
   var x;
@@ skipping 99 matching lines @@
 }"""),
   const TestEntry(r"""
 class C<T> {
   foo() => T;
 }
 main() {
   print(new C<int>().foo());
 }""", r"""
 function() {
   P.print(V.C$(P.$int).foo$0());
-  return null;
 }"""),
   const TestEntry(r"""
 class C<T> {
   foo() => C;
 }
 main() {
   print(new C<int>().foo());
 }""", r"""
 function() {
   P.print(V.C$().foo$0());
-  return null;
 }"""),
   const TestEntry.forMethod('generative_constructor(C#)', r"""
 class C<T> {
   C() { print(T); }
   foo() => print(T);
 }
 main() {
   new C<int>();
 }""", r"""
 function($T) {
@@ skipping 51 matching lines @@
 class Foo {
   factory Foo.make(x) = Foo.create;
   var x;
   Foo.create(this.x);
 }
 main() {
   print(new Foo.make(5));
 }""", r"""
 function() {
   P.print(V.Foo$create(5));
-  return null;
 }"""),
 const TestEntry(r"""
 class A {
   factory A(x) = B<int>;
   get typevar;
 }
 class B<T> implements A {
   var x;
   B(this.x);
 
   get typevar => T;
 }
 main() {
   new A(5).typevar;
 }""", r"""
 function() {
   V.B$(5, P.$int).get$typevar();
-  return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }