[snapshot] Move builtins generation into mksnapshot

src/api.h

Index: src/api.h
diff --git a/src/api.h b/src/api.h
index 7be1e6727351b46168cb287412b5ccdb03a8c13d..4243d9025952e9e1366824763d0a23792fe02015 100644
--- a/src/api.h
+++ b/src/api.h
@@ -347,6 +347,8 @@ OPEN_HANDLE_LIST(MAKE_OPEN_HANDLE)
 #undef MAKE_OPEN_HANDLE
 #undef OPEN_HANDLE_LIST
 
+extern Isolate* IsolateNewImpl(internal::Isolate* isolate,
+                               const Isolate::CreateParams& params);
 
 namespace internal {
 
@@ -645,7 +647,6 @@ void HandleScopeImplementer::DeleteExtensions(internal::Object** prev_limit) {
          (!blocks_.is_empty() && prev_limit != NULL));
 }
 
-
 // Interceptor functions called from generated inline caches to notify
 // CPU profiler that external callbacks are invoked.
 void InvokeAccessorGetterCallback(