Optimize is-check to instanceof when it eliminates an interceptor

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 07ee0c0bceeadbd2743adbc7661bb7b97a1ce0a7..1016cf33cfa74ec02c0fe2c66b111beb894ae103 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -3442,7 +3442,8 @@ class SsaBuilder extends ResolvedVisitor {
       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);
     }