Fix top-level inference errors for most of the newly failing tests.

tests/language_strong/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.
 
 // Regression test for dart2js that used to not see a closure could be
 // invoked through a getter access followed by an invocation.
 
-var closure = (a) => a.toString();
+var closure = (Object a) => a.toString();
 
 get foo => closure;
 
 main() {
   if (foo(42) != '42') {
     throw 'Test failed';
   }
 }