Move StartIsolateScope into MessageHandler::HandleMessages

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 269ff6c5d973ea26610b39b4a0a182bf04a91e06..3019f479da0a48c6f0b8cc2e0bb8b45b0e20ca16 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -762,7 +762,10 @@ class StartIsolateScope {
  public:
   explicit StartIsolateScope(Isolate* new_isolate)
       : new_isolate_(new_isolate), saved_isolate_(Isolate::Current()) {
-    ASSERT(new_isolate_ != NULL);
+    if (new_isolate_ == NULL) {
+      // Do nothing.
+      return;
+    }
     if (saved_isolate_ != new_isolate_) {
       ASSERT(Isolate::Current() == NULL);
       Isolate::SetCurrent(new_isolate_);
@@ -771,6 +774,10 @@ class StartIsolateScope {
   }
 
   ~StartIsolateScope() {
+    if (new_isolate_ == NULL) {
+      // Do nothing.
+      return;
+    }
     if (saved_isolate_ != new_isolate_) {
       new_isolate_->ClearStackLimit();
       Isolate::SetCurrent(saved_isolate_);