Fix background compilation: allocate stubs at safepoint

runtime/vm/virtual_memory_macos.cc

Index: runtime/vm/virtual_memory_macos.cc
diff --git a/runtime/vm/virtual_memory_macos.cc b/runtime/vm/virtual_memory_macos.cc
index f28da352ef30940154baf3b98c23931e187c36ad..484ad7876616fda5d25850acc08d68711fe162d0 100644
--- a/runtime/vm/virtual_memory_macos.cc
+++ b/runtime/vm/virtual_memory_macos.cc
@@ -13,6 +13,8 @@
 #include "platform/assert.h"
 #include "platform/utils.h"
 
+#include "vm/isolate.h"
+
 namespace dart {
 
 // standard MAP_FAILED causes "error: use of old-style cast" as it
@@ -80,6 +82,8 @@ bool VirtualMemory::Commit(uword addr, intptr_t size, bool executable) {
 
 
 bool VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {
+  ASSERT(Thread::Current()->IsMutatorThread() ||
+         Isolate::Current()->mutator_thread()->IsAtSafepoint());
   uword start_address = reinterpret_cast<uword>(address);
   uword end_address = start_address + size;
   uword page_address = Utils::RoundDown(start_address, PageSize());