Optimize is-check to instanceof when it eliminates an interceptor

pkg/compiler/lib/src/ssa/builder.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 ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   SsaCodeGeneratorTask generator;
   SsaBuilderTask builder;
   SsaOptimizerTask optimizer;
@@ skipping 3424 matching lines @@
       return new HIs.compound(type, expression, call, backend.boolType);
     } else if (type.isMalformed) {
       ErroneousElement element = type.element;
       generateTypeError(node, element.message);
       HInstruction call = pop();
       return new HIs.compound(type, expression, call, backend.boolType);
     } else {
       if (backend.hasDirectCheckFor(type)) {
         return new HIs.direct(type, expression, backend.boolType);
       }
-      // TODO(johnniwinther): Avoid interceptor if unneeded.
+      // The interceptor is not always needed.  It is removed by optimization
+      // when the receiver type or tested type permit.
       return new HIs.raw(
           type, expression, invokeInterceptor(expression), backend.boolType);
     }
   }
 
   HInstruction buildFunctionType(FunctionType type) {
     type.accept(new TypeBuilder(compiler.world), this);
     return pop();
   }
 
@@ skipping 3214 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }