Make sky/tools/packager build on Mac

sky/tools/packager/vm.cc

Index: sky/tools/packager/vm.cc
diff --git a/sky/tools/packager/vm.cc b/sky/tools/packager/vm.cc
index 5e45eb8cb21fea7b0b71d51bf2b89637af0bb662..a36a70fe579988830ff4db22a0ea7a13edd35ab3 100644
--- a/sky/tools/packager/vm.cc
+++ b/sky/tools/packager/vm.cc
@@ -26,7 +26,7 @@ void InitDartVM() {
 Dart_Isolate CreateDartIsolate() {
   CHECK(blink::kDartIsolateSnapshotBuffer);
   char* error = nullptr;
-  Dart_Isolate isolate = Dart_CreateIsolate("http://example.com", "main",
+  Dart_Isolate isolate = Dart_CreateIsolate("dart:snapshot", "main",
                                             blink::kDartIsolateSnapshotBuffer,
                                             nullptr, nullptr, &error);