Migrate NoSafepointScope; add constrained concurrent allocation to unit test

runtime/vm/raw_object_snapshot.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 2041 matching lines @@
                                           intptr_t object_id,
                                           intptr_t tags,
                                           Snapshot::Kind kind) {
   // Read the length so that we can determine instance size to allocate.
   ASSERT(reader != NULL);
   intptr_t len = reader->ReadSmiValue();
   intptr_t hash = reader->ReadSmiValue();
   String& str_obj = String::Handle(reader->zone(), String::null());
 
   if (kind == Snapshot::kFull) {
-    ASSERT(reader->isolate()->no_safepoint_scope_depth() != 0);
+    // We currently only expect the Dart mutator to read snapshots.
+    reader->isolate()->AssertCurrentThreadIsMutator();
+    ASSERT(Thread::Current()->no_safepoint_scope_depth() != 0);
     RawOneByteString* obj = reader->NewOneByteString(len);
     str_obj = obj;
     str_obj.set_tags(tags);
     str_obj.SetHash(hash);
     if (len > 0) {
       uint8_t* raw_ptr = CharAddr(str_obj, 0);
       reader->ReadBytes(raw_ptr, len);
     }
     ASSERT((hash == 0) || (String::Hash(str_obj, 0, str_obj.Length()) == hash));
   } else {
@@ skipping 1024 matching lines @@
     // We do not allow objects with native fields in an isolate message.
     writer->SetWriteException(Exceptions::kArgument,
                               "Illegal argument in isolate message"
                               " : (object is a UserTag)");
   } else {
     UNREACHABLE();
   }
 }
 
 }  // namespace dart