Fix multi-threaded access to logs

runtime/vm/log.cc

 // Copyright (c) 2015, 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/log.h"
 
 #include "vm/flags.h"
 #include "vm/thread.h"
 
 namespace dart {
@@ skipping 123 matching lines @@
 }
 
 
 Log Log::noop_log_;
 Log* Log::NoOpLog() {
   return &noop_log_;
 }
 
 
 void Log::TerminateString() {
+  if (this == NoOpLog()) {
+    return;
+  }
   buffer_.Add('\0');
 }
 
 
 void Log::EnableManualFlush() {
+  if (this == NoOpLog()) {
+    return;
+  }
   manual_flush_++;
 }
 
 
 void Log::DisableManualFlush() {
+  if (this == NoOpLog()) {
+    return;
+  }
+
   manual_flush_--;
   ASSERT(manual_flush_ >= 0);
   if (manual_flush_ == 0) {
     Flush();
   }
 }
 
 
 void LogBlock::Initialize() {
   log_->EnableManualFlush();
 }
 
 
 LogBlock::~LogBlock() {
   log_->Flush(cursor_);
   log_->DisableManualFlush();
 }
 
 }  // namespace dart