Enable downward inference on default params

test/checker/checker_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.
 
 /// General type checking tests
 library dev_compiler.test.checker_test;
 
 import 'package:unittest/unittest.dart';
 
 import 'package:dev_compiler/src/testing.dart';
@@ skipping 149 matching lines @@
           T foo() => /*warning:DownCastImplicit*/null;
 
           void bar() {
             int /*severe:InvalidVariableDeclaration*/x;
             num y;
             // TODO(vsm): This should be a runtime check:
             // Transformed to: T z = cast(null, T)
             T /*severe:InvalidVariableDeclaration*/z;
           }
 
-          void baz(T x, [T /*severe:InvalidVariableDeclaration*/y, T z = /*severe:StaticTypeError*/null]) {
+          void baz(T x, [T /*severe:InvalidVariableDeclaration*/y, T z = /*warning:DownCastImplicit*/null]) {
           }
         }
 
         class B<T extends List> {
           T x;
 
           // T cannot be primitive.
           T foo() => null;
         }
 
@@ skipping 2651 matching lines @@
             f = bar as II2D;
             f = bar as DD2I;
             f = bar as DI2D;
             f = bar as ID2D;
             f = bar as DD2D;
           }
       '''
     });
   });
 }