dart2js cps: Support function types in 'is' and 'as' operators.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 6c2e126ca890bd53239698b05f65c12321daea08..e037ff376d1b22f1561d66a128f0f6790b83cff1 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1185,6 +1185,14 @@ class JavaScriptBackend extends Backend {
     }
   }
 
+  /// Call during codegen if an instance of [closure] is being created.
+  void registerInstantiatedClosure(LocalFunctionElement closure,
+                                   CodegenRegistry registry) {
+    if (methodNeedsRti(closure)) {
+      registerComputeSignature(compiler.enqueuer.codegen, registry);
+    }
+  }
+
   void registerBoundClosure(Enqueuer enqueuer) {
     boundClosureClass.ensureResolved(compiler);
     enqueuer.registerInstantiatedType(