[snapshot] Move builtins generation into mksnapshot

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 43be499b3d88ff78ce7c5d16791a7dcac0ee5c03..5c27b69135f34618d8b9727cc3ea0cf9ed343944 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -8181,6 +8181,12 @@ Isolate* Isolate::GetCurrent() {
 
 Isolate* Isolate::New(const Isolate::CreateParams& params) {
   i::Isolate* isolate = new i::Isolate(false);
+  return IsolateNewImpl(isolate, params);
+}
+
+// This is separate so that tests can provide a different |isolate|.
+Isolate* IsolateNewImpl(internal::Isolate* isolate,
+                        const v8::Isolate::CreateParams& params) {
   Isolate* v8_isolate = reinterpret_cast<Isolate*>(isolate);
   CHECK(params.array_buffer_allocator != NULL);
   isolate->set_array_buffer_allocator(params.array_buffer_allocator);