Fixes crashes in VM isolate shutdown.

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index d49b7f4b88d8f6de906b8b8ff191a0ce90b31fb5..a8c0275d5a64a3ea9f9af27a87d4279c9b6a6f33 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -131,6 +131,7 @@ const char* Dart::InitOnce(const uint8_t* vm_isolate_snapshot,
                0,  // New gen size 0; VM isolate should only allocate in old.
                FLAG_old_gen_heap_size * MBInWords,
                FLAG_external_max_size * MBInWords);
+    Object::InitNull(vm_isolate_);
     ObjectStore::Init(vm_isolate_);
     TargetCPUFeatures::InitOnce();
     Object::InitOnce(vm_isolate_);
@@ -315,6 +316,7 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
   ServiceIsolate::MaybeInjectVMServiceLibrary(isolate);
 
   ServiceIsolate::SendIsolateStartupMessage();
+  ASSERT(isolate->debugger() != NULL);
   isolate->debugger()->NotifyIsolateCreated();
 
   // Create tag table.