dart2js cps: Translate identity placeholders to refinement nodes.

pkg/compiler/lib/src/js/js.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.
 
 library js;
 
 import 'package:js_ast/js_ast.dart';
 export 'package:js_ast/js_ast.dart';
 
 import '../common.dart';
@@ skipping 144 matching lines @@
           }
         }
       }
       _cachedLiteral = js.escapedString(text);
     }
     return _cachedLiteral;
   }
 
   @override
   String get value => _literal.value;
-}
+}
+
+/// True if the given template consists of just a placeholder. Such templates
+/// are sometimes used to manually promote the type of an expression.
+bool isIdentityTemplate(Template template) {
+  return template.source == '#';

[sra1, 2016/01/11 19:31:20]

It would be better to check the ast for a placeholder, e.g. how it is done in
https://chromiumcodereview.appspot.com/1567103006/

[asgerf, 2016/01/11 20:11:44]

Replaced with an is-check.

+}