dart2js cps_ir: Use interceptors for is-checks.

pkg/compiler/lib/src/cps_ir/insert_refinements.dart

Index: pkg/compiler/lib/src/cps_ir/insert_refinements.dart
diff --git a/pkg/compiler/lib/src/cps_ir/insert_refinements.dart b/pkg/compiler/lib/src/cps_ir/insert_refinements.dart
index afea3976dee6bb0de887eee47d1d8de6b271ba79..5922247f4c9b975a8730bdee93bd7dfb9eb78eca 100644
--- a/pkg/compiler/lib/src/cps_ir/insert_refinements.dart
+++ b/pkg/compiler/lib/src/cps_ir/insert_refinements.dart
@@ -161,6 +161,14 @@ class InsertRefinements extends RecursiveVisitor implements Pass {
       push(falseCont);
       return;
     }
+    if (condition is TypeTestRaw) {
+      Primitive value = condition.value.definition;
+      TypeMask type = types.subtypesOf(condition.dartType);
+      Primitive refinedValue = new Refinement(value, type);
+      pushRefinement(trueCont, refinedValue);
+      push(falseCont);
+      return;
+    }
 
     // If the condition is comparison with a constant, promote the other value.
     // This can happen either for calls to `==` or `identical` calls, such