Make microtask queue be per thread (flutter/engine#3644)

Previously, it was a static. Doesn't have much effect in practice because we
only use this microtask queue for main isolates, which exist on a single
thread.

DEPS

@@ -57,7 +57,7 @@ deps = {
    Var('fuchsia_git') + '/ftl' + '@' + 'f1357b6eaa9a23cffec1645dfeba610b5f926b1d',
 
   'src/lib/tonic':
-   Var('fuchsia_git') + '/tonic' + '@' + '82075233dc0cd23b1c8af5f694eb86dd9bd10f35',
+   Var('fuchsia_git') + '/tonic' + '@' + '02431161f012a18c32f885e04b1eda51e6c11f8e',
 
   'src/lib/zip':
    Var('fuchsia_git') + '/zip' + '@' + '92dc87ca645fe8e9f5151ef6dac86d8311a7222f',

engine/src/flutter/lib/ui/dart_runtime_hooks.cc

@@ -172,7 +172,7 @@ void ScheduleMicrotask(Dart_NativeArguments args) {
   Dart_Handle closure = Dart_GetNativeArgument(args, 0);
   if (LogIfError(closure) || !Dart_IsClosure(closure))
     return;
-  tonic::DartMicrotaskQueue::ScheduleMicrotask(closure);
+  tonic::DartMicrotaskQueue::GetForCurrentThread()->ScheduleMicrotask(closure);
 }
 
 }  // namespace blink

engine/src/flutter/lib/ui/window/window.cc

@@ -223,7 +223,7 @@ void Window::BeginFrame(ftl::TimePoint frameTime) {
                       Dart_NewInteger(microseconds),
                   });
 
-  tonic::DartMicrotaskQueue::RunMicrotasks();
+  tonic::DartMicrotaskQueue::GetForCurrentThread()->RunMicrotasks();
 
   DartInvokeField(library_.value(), "_drawFrame", {});
 }

engine/src/flutter/shell/platform/linux/main_linux.cc

@@ -103,7 +103,7 @@ void RunNonInteractive(ftl::CommandLine initial_command_line,
   if (error == tonic::kNoError)
     error = task_observer.last_error();
   if (error == tonic::kNoError)
-    error = tonic::DartMicrotaskQueue::GetLastError();
+    error = tonic::DartMicrotaskQueue::GetForCurrentThread()->GetLastError();
 
   // The script has completed and the engine may not be in a clean state,
   // so just stop the process.