dart2js: Remove obsolete shortcut code in function-type checking.

sdk/lib/_internal/compiler/js_lib/js_rti.dart

Index: sdk/lib/_internal/compiler/js_lib/js_rti.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_rti.dart b/sdk/lib/_internal/compiler/js_lib/js_rti.dart
index 7246aa49c9992fa294f47132e771dd4ba64242e3..8ffd6e0c609a2a43341876a9ba68ea4b07d76657 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_rti.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_rti.dart
@@ -400,9 +400,6 @@ bool checkSubtypeOfRuntimeType(o, t) {
   } else if (hasField(t, '${JS_FUNCTION_TYPE_TAG()}')) {
     // Functions are treated specially and have their type information stored
     // directly in the instance.
-    var signatureName =
-        '${JS_OPERATOR_IS_PREFIX()}_${getField(t, JS_FUNCTION_TYPE_TAG())}';
-    if (hasField(o, signatureName)) return true;
     var targetSignatureFunction = getField(o, '${JS_SIGNATURE_NAME()}');
     if (targetSignatureFunction == null) return false;
     type = invokeOn(targetSignatureFunction, o, null);