Fix timeline time origin / extent printing with an empty timeline

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 <cstdlib>
 
 #include "vm/atomic.h"
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/lockers.h"
@@ skipping 1103 matching lines @@
   if (time_high_micros_ < micros) {
     time_high_micros_ = micros;
   }
   if (time_low_micros_ > micros) {
     time_low_micros_ = micros;
   }
 }
 
 
 int64_t TimelineEventRecorder::TimeOriginMicros() const {
+  if (time_high_micros_ == 0) {
+    return 0;
+  }
   return time_low_micros_;
 }
 
 
 int64_t TimelineEventRecorder::TimeExtentMicros() const {
+  if (time_high_micros_ == 0) {
+    return 0;
+  }
   return time_high_micros_ - time_low_micros_;
 }
 
 
 void TimelineEventRecorder::ThreadBlockCompleteEvent(TimelineEvent* event) {
   if (event == NULL) {
     return;
   }
   // Grab the current thread.
   OSThread* thread = OSThread::Current();
@@ skipping 549 matching lines @@
 TimelineEventBlock* TimelineEventBlockIterator::Next() {
   ASSERT(current_ != NULL);
   TimelineEventBlock* r = current_;
   current_ = current_->next();
   return r;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart