Use a monotonic clock in the implementation of Stopwatch.

runtime/vm/dart_api_impl.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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "vm/class_finalizer.h"
@@ skipping 5542 matching lines @@
     return Api::NewError("%s expects argument 'bytes_length' to be >= 0.",
                          CURRENT_FUNC);
   }
   Service::SendEmbedderEvent(I, stream_id, event_kind,
                              bytes, bytes_length);
   return Api::Success();
 }
 
 
 DART_EXPORT int64_t Dart_TimelineGetMicros() {
-  return OS::GetCurrentTraceMicros();
+  return OS::GetCurrentMonotonicMicros();
 }
 
 
 DART_EXPORT void Dart_TimelineSetRecordedStreams(int64_t stream_mask) {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);
   isolate->GetAPIStream()->set_enabled(
       (stream_mask & DART_TIMELINE_STREAM_API) != 0);
   isolate->GetCompilerStream()->set_enabled(
       (stream_mask & DART_TIMELINE_STREAM_COMPILER) != 0);
@@ skipping 353 matching lines @@
                                    ApiReallocate);
   writer.WriteFullSnapshot();
   *vm_isolate_snapshot_size = writer.VmIsolateSnapshotSize();
   *isolate_snapshot_size = writer.IsolateSnapshotSize();
   *instructions_snapshot_size = writer.InstructionsSnapshotSize();
 
   return Api::Success();
 }
 
 }  // namespace dart