Fix bug in the closure methods, where iterating over the boxed variable did not return the variable…

tests/compiler/dart2js/simple_inferrer_closure_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart' show buildSourceString;
 
 const String TEST = """
@@ skipping 65 matching lines @@
       var b = {};
       b = 42;
       return b;
     }
     a = g();
   } finally {
   }
   return a;
 }
 
+returnIntOrNull() {
+  for (var b in [42]) {
+    var bar = 42;
+    f() => bar;
+    bar = null;
+    return f();
+  }
+  return 42;
+}
+
 class A {
   foo() {
     f() => this;
     return f();
   }
 }
 
 main() {
   returnInt1();
   returnDyn1();
   returnInt2();
   returnDyn2();
   returnInt3();
   returnDyn3();
   returnInt4();
   returnNum1();
+  returnIntOrNull();
   new A().foo();
 }
 """;
 
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkReturn(String name, type) {
       var element = findElement(compiler, name);
       Expect.equals(type,
           typesInferrer.getReturnTypeOfElement(element).simplify(compiler));
     }
 
     checkReturn('returnInt1', compiler.typesTask.intType);
     checkReturn('returnInt2', compiler.typesTask.intType);
     checkReturn('returnInt3', compiler.typesTask.intType);
     checkReturn('returnInt4', compiler.typesTask.intType);
+    checkReturn('returnIntOrNull', compiler.typesTask.intType.nullable());
 
     checkReturn('returnDyn1', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn2', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn3', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnNum1', compiler.typesTask.numType);
 
     checkReturnInClass(String className, String methodName, type) {
       var cls = findElement(compiler, className);
       var element = cls.lookupLocalMember(buildSourceString(methodName));
       Expect.equals(type,
           typesInferrer.getReturnTypeOfElement(element).simplify(compiler));
     }
     var cls = findElement(compiler, 'A');
     checkReturnInClass('A', 'foo', new TypeMask.nonNullExact(cls.rawType));
   }));
 }