Use predicates to check simple function types.

sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart b/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
index 29e1ecba526e5e4e8c4f79a4438fdf5e2cb4ab2f..9308becabd06b2db80a661a694cacfc327fa2637 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
@@ -545,6 +545,18 @@ class RuntimeTypes {
     return representationGenerator.getTypeRepresentation(type, onVariable);
   }
 
+  bool isSimpleFunctionType(FunctionType type) {
+    if (!type.returnType.isDynamic) return false;
+    if (!type.optionalParameterTypes.isEmpty) return false;
+    if (!type.namedParameterTypes.isEmpty) return false;
+    for (Link<DartType> link = type.parameterTypes;
+        !link.isEmpty;
+        link = link.tail) {
+      if (!link.head.isDynamic) return false;
+    }
+    return true;
+  }
+
   static bool hasTypeArguments(DartType type) {
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;