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

tests/compiler/dart2js/js_backend_cps_ir_control_flow_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 control flow statements.
 
 library control_flow_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 42 matching lines @@
     print(1);
     if (foo(false)) break;
   }
   print(2);
 }""", """
 function() {
   var i = 0;
   L1:
     while (P.identical(V.foo(true), true)) {
       P.print(1);
-      if (!P.identical(V.foo(false), true))
-        i = V.foo(i);
-      else
+      if (P.identical(V.foo(false), true))
         break L1;
+      i = V.foo(i);
     }
   P.print(2);
-  return null;
 }"""),
 const TestEntry("""
 foo(a) => a;
 
 main() {
  if (foo(true)) {
    print(1);
  } else {
    print(2);
  }
  print(3);
 }""", """
 function() {
   V.foo(true) ? P.print(1) : P.print(2);
   P.print(3);
-  return null;
 }"""),
 const TestEntry("""
 foo(a) => a;
 
 main() {
  if (foo(true)) {
    print(1);
    print(1);
  } else {
    print(2);
    print(2);
  }
  print(3);
 }""", """
 function() {
   if (V.foo(true)) {
     P.print(1);
     P.print(1);
   } else {
     P.print(2);
     P.print(2);
   }
   P.print(3);
-  return null;
 }"""),
 const TestEntry("""
 main() {
   if (1) {
     print('bad');
   } else {
     print('good');
   }
 }""","""
 function() {
   P.print("good");
-  return null;
 }"""),
   const TestEntry("""
 foo() => 2;
 main() {
   if (foo()) {
     print('bad');
   } else {
     print('good');
   }
 }""","""
 function() {
   V.foo();
   P.print("good");
-  return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }