dart2js: add classNameFromIsCheckProperty to compiler builtins.

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

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index 240e771d063c3f3c9bb481923807fa056be259bd..3a02c677e06608054777c32f0b44038b3c4f3172 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -276,6 +276,10 @@ class OldEmitter implements Emitter {
         return jsAst.js.expressionTemplateYielding(
             typeAccess(compiler.objectClass));
 
+      case JsBuiltin.classNameFromIsCheckProperty:
+        int isPrefixLength = namer.operatorIsPrefix.length;
+        return jsAst.js.expressionTemplateFor('#.substring($isPrefixLength)');
+
       case JsBuiltin.isFunctionType:
         return backend.rti.representationGenerator.templateForIsFunctionType;