dart2js: add builtin 'isSubtype'.

pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart

 // Copyright (c) 2014, 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 dart2js.new_js_emitter.emitter;
 
 import 'package:_internal/compiler/js_lib/shared/embedded_names.dart' show
     JsBuiltin;
 
 import '../program_builder.dart' show ProgramBuilder;
@@ skipping 144 matching lines @@
       case JsBuiltin.typeName:
         return js.js.expressionTemplateFor("#.$typeNameProperty");
 
       case JsBuiltin.rawRuntimeType:
         return js.js.expressionTemplateFor("#.constructor");
 
       case JsBuiltin.createFunctionType:
         return _backend.rti.representationGenerator
             .templateForCreateFunctionType;
 
+      case JsBuiltin.isSubtype:
+        // TODO(floitsch): move this closer to where is-check properties are
+        // built.
+        String isPrefix = namer.operatorIsPrefix;
+        return js.js.expressionTemplateFor("('$isPrefix' + #) in #.prototype");
+
       default:
         _compiler.internalError(NO_LOCATION_SPANNABLE,
                                 "Unhandled Builtin: $builtin");
         return null;
     }
   }
 
   @override
   void invalidateCaches() {
   }
 }