use async_get_default_dispatcher instead of creating a Loop object

Author: George Wright

shell/platform/fuchsia/flutter/main.cc

@@ -22,10 +22,9 @@ int main(int argc, char const* argv[]) {
   {
     TRACE_DURATION("flutter", "CreateTraceProvider");
     bool already_started;
-    async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
     // Use CreateSynchronously to prevent loss of early events.
     trace::TraceProviderWithFdio::CreateSynchronously(
-        loop.dispatcher(), "flutter_runner", &provider, &already_started);
+        async_get_default_dispatcher(), "flutter_runner", &provider, &already_started);
   }
 
   // Set up the process-wide /tmp memfs.

shell/platform/fuchsia/flutter/runner.cc

@@ -284,8 +284,7 @@ bool Runner::SetupTZDataInternal() {
 void Runner::SetupTraceObserver() {
   trace_observer_ = std::make_unique<trace::TraceObserver>();
   ASSERT_TRUE(message_loop_.GetTaskRunner()->RunsTasksOnCurrentThread());
-  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
-  trace_observer_->Start(loop.dispatcher(), [runner = this]() {
+  trace_observer_->Start(async_get_default_dispatcher()), [runner = this]() {
     if (!trace_is_category_enabled("dart:profiler")) {
       return;
     }