Add a SafepointMutexLocker class so that it is possible to have scopes inside the MutexLocker which…

runtime/vm/symbols.cc

Index: runtime/vm/symbols.cc
diff --git a/runtime/vm/symbols.cc b/runtime/vm/symbols.cc
index f6735c232dd3bf00f4ec306e6829dec51113fd0d..d35dc6d20e4a5096fdb144f1b2d62334e841bc2a 100644
--- a/runtime/vm/symbols.cc
+++ b/runtime/vm/symbols.cc
@@ -564,6 +564,7 @@ RawString* Symbols::NewSymbol(const StringType& str) {
     table.Release();
   }
   if (symbol.IsNull()) {
+    SafepointMutexLocker ml(isolate->symbols_mutex());
     SymbolTable table(zone, isolate->object_store()->symbol_table());
     symbol ^= table.InsertNewOrGet(str);
     isolate->object_store()->set_symbol_table(table.Release());
@@ -587,6 +588,7 @@ RawString* Symbols::Lookup(const StringType& str) {
     table.Release();
   }
   if (symbol.IsNull()) {
+    SafepointMutexLocker ml(isolate->symbols_mutex());
     SymbolTable table(zone, isolate->object_store()->symbol_table());
     symbol ^= table.GetOrNull(str);
     table.Release();