Add type inference of annotations.

pkg/front_end/testcases/inference/downwards_inference_annotations.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 class Foo {
   const Foo(List<String> l);
   const Foo.named(List<String> l);
 }
 
 @Foo(/*@typeArgs=String*/ const [])
 class Bar {}
 
 @Foo.named(/*@typeArgs=String*/ const [])
 class Baz {}
+
+main() {}