Add a TurboFan skeleton for StringAddStub.

test/cctest/compiler/test-run-stubs.cc

Index: test/cctest/compiler/test-run-stubs.cc
diff --git a/test/cctest/compiler/test-run-stubs.cc b/test/cctest/compiler/test-run-stubs.cc
index 5c012a3301f56aae43b82080631a3844d214576b..5baa79a57ac531e1df9f59780670806841b3670b 100644
--- a/test/cctest/compiler/test-run-stubs.cc
+++ b/test/cctest/compiler/test-run-stubs.cc
@@ -106,4 +106,42 @@ TEST(RunStringLengthTFStub) {
   CHECK_EQ(static_cast<int>(strlen(testString)), Smi::cast(*result)->value());
 }
 
+
+TEST(RunStringAddTFStub) {
+  HandleAndZoneScope scope;
+  Isolate* isolate = scope.main_isolate();
+  Zone* zone = scope.main_zone();
+
+  // Create code and an accompanying descriptor.
+  StringAddTFStub stub(isolate, STRING_ADD_CHECK_BOTH, NOT_TENURED);
+  Handle<Code> code = stub.GenerateCode();
+  CompilationInfo info(&stub, isolate, zone);
+  CallDescriptor* descriptor = Linkage::ComputeIncoming(zone, &info);
+
+  // Create a function to call the code using the descriptor.
+  Graph graph(zone);
+  CommonOperatorBuilder common(zone);
+  // FunctionTester (ab)uses a 2-argument function
+  Node* start = graph.NewNode(common.Start(2));
+  // Parameter 0 is the receiver
+  Node* leftParam = graph.NewNode(common.Parameter(1), start);
+  Node* rightParam = graph.NewNode(common.Parameter(2), start);
+  Unique<HeapObject> u = Unique<HeapObject>::CreateImmovable(code);
+  Node* theCode = graph.NewNode(common.HeapConstant(u));
+  Node* dummyContext = graph.NewNode(common.NumberConstant(0.0));
+  Node* call = graph.NewNode(common.Call(descriptor), theCode, leftParam,
+                             rightParam, dummyContext, start, start);
+  Node* ret = graph.NewNode(common.Return(), call, call, start);
+  Node* end = graph.NewNode(common.End(), ret);
+  graph.SetStart(start);
+  graph.SetEnd(end);
+  FunctionTester ft(&graph);
+
+  // Actuall call through to the stub, verifying its result.
+  Handle<String> leftArg = ft.Val("links");
+  Handle<String> rightArg = ft.Val("rechts");
+  Handle<Object> result = ft.Call(leftArg, rightArg).ToHandleChecked();
+  CHECK(String::Equals(ft.Val("linksrechts"), Handle<String>::cast(result)));
+}
+
 #endif  // V8_TURBOFAN_TARGET