dart2js: add builtin 'isSubtype'.

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

Index: pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
index a700946fb1617c01a6f1c25fb32a88a685ee9077..1e2dae36dd30550a0a21543043df06ae8b3db5d6 100644
--- a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
@@ -162,6 +162,12 @@ class Emitter implements emitterTask.Emitter {
         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");