Optimize is-check to instanceof when it eliminates an interceptor

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 const bool USE_CPS_IR = const bool.fromEnvironment("USE_CPS_IR");
 
@@ skipping 1452 matching lines @@
         element == compiler.boolClass ||
         element == compiler.numClass ||
         element == compiler.intClass ||
         element == compiler.doubleClass ||
         element == jsArrayClass ||
         element == jsMutableArrayClass ||
         element == jsExtendableArrayClass ||
         element == jsFixedArrayClass;
   }
 
+  bool mayGenerateInstanceofCheck(DartType type) {
+    // We can use an instanceof check for raw types that have no subclass that
+    // is mixed-in or in an implements clause.
+
+    if (!type.isRaw) return false;
+    ClassElement classElement = type.element;
+    if (isInterceptorClass(classElement)) return false;
+    return compiler.world.hasOnlySubclasses(classElement);
+  }
+
   Element getExceptionUnwrapper() {
     return findHelper('unwrapException');
   }
 
   Element getThrowRuntimeError() {
     return findHelper('throwRuntimeError');
   }
 
   Element getThrowTypeError() {
     return findHelper('throwTypeError');
@@ skipping 1032 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }