Implement spawnUri from dart:isolate. This function allows us to

runtime/vm/benchmark_test.cc

Index: runtime/vm/benchmark_test.cc
===================================================================
--- runtime/vm/benchmark_test.cc	(revision 7714)
+++ runtime/vm/benchmark_test.cc	(working copy)
@@ -56,7 +56,7 @@
   const int kNumIterations = 100;
   char* err = NULL;
   Dart_Isolate base_isolate = Dart_CurrentIsolate();
-  Dart_Isolate test_isolate = Dart_CreateIsolate(NULL, NULL, NULL, &err);
+  Dart_Isolate test_isolate = Dart_CreateIsolate(NULL, NULL, NULL, NULL, &err);
   EXPECT(test_isolate != NULL);
   Dart_EnterScope();
   uint8_t* buffer = NULL;
@@ -66,7 +66,8 @@
   Timer timer(true, "Core Isolate startup benchmark");
   timer.Start();
   for (int i = 0; i < kNumIterations; i++) {
-    Dart_Isolate new_isolate = Dart_CreateIsolate(NULL, buffer, NULL, &err);
+    Dart_Isolate new_isolate =
+        Dart_CreateIsolate(NULL, NULL, buffer, NULL, &err);
     EXPECT(new_isolate != NULL);
     Dart_ShutdownIsolate();
   }