Fixes build for uninitialized local variable.

runtime/vm/timeline.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 "platform/globals.h"
 #ifndef PRODUCT
 
 #include "vm/timeline.h"
 
 #include <errno.h>
@@ skipping 1695 matching lines @@
                                                      int64_t type,
                                                      int64_t flow_id,
                                                      const char* args) {
   const int64_t pid = OS::ProcessId();
   OSThread* os_thread = thread->os_thread();
   ASSERT(os_thread != NULL);
   const int64_t tid = OSThread::ThreadIdToIntPtr(os_thread->trace_id());
 
   TimelineEvent::EventType event_type =
       static_cast<TimelineEvent::EventType>(type);
-  const char* typestr;
+  const char* typestr = NULL;
   const char* bpstr = "";
   switch (event_type) {
     case TimelineEvent::kFlowBegin:
       typestr = "s";
       break;
     case TimelineEvent::kFlowStep:
       typestr = "t";
       break;
     case TimelineEvent::kFlowEnd:
       typestr = "f";
@@ skipping 40 matching lines @@
                            name, category, tid, pid, start, args);
 
   event->Instant("", start);
   // json was allocated in the zone and a copy will be stored in event.
   event->CompleteWithPreSerializedJSON(json);
 }
 
 }  // namespace dart
 
 #endif  // !PRODUCT