don't use default isolate in mksnapshot

src/serialize.cc

Index: src/serialize.cc
diff --git a/src/serialize.cc b/src/serialize.cc
index d05dd2612270c8e16f3f0c7d0354d94eaddbe49a..b3a78780a9413da5d90e5403533bea82326da410 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -1284,7 +1284,6 @@ Serializer::Serializer(Isolate* isolate, SnapshotByteSink* sink)
       root_index_wave_front_(0) {
   // The serializer is meant to be used only to generate initial heap images
   // from a context in which there is only one isolate.
-  ASSERT(isolate_->IsDefaultIsolate());
   for (int i = 0; i <= LAST_SPACE; i++) {
     fullness_[i] = 0;
   }