Fix compiler warning

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 900 matching lines @@
 
 
 bool Isolate::FetchStackFrameDetails() {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate->stack_frame_index_ >= 0);
   MonitorLocker ml(status_sync);
   DebuggerStackTrace* stack = Debugger::CollectStackTrace();
   intptr_t frame_index = isolate->stack_frame_index_;
   if (frame_index >= stack->Length()) {
     // Frame no longer available.
-    return NULL;
+    return false;
   }
   ActivationFrame* frame = stack->FrameAt(frame_index);
   TextBuffer buffer(256);
   buffer.Printf("{ \"handle\": \"0x%" Px64 "\", \"frame_index\": %" Pd ", ",
        reinterpret_cast<int64_t>(isolate), frame_index);
 
   const Code& code = frame->code();
   buffer.Printf("\"code\": { \"size\": %" Pd ", ", code.Size());
   buffer.Printf("\"alive\": %s, ", code.is_alive() ? "false" : "true");
   buffer.Printf("\"optimized\": %s }, ",
@@ skipping 253 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart