Make drain() consistently asynchronous.

Author: Hixie

lib/ui/channel_buffers.dart

@@ -181,7 +181,11 @@ class ChannelBuffers {
   ///
   /// This should be called once a channel is prepared to handle messages
   /// (i.e. when a message handler is setup in the framework).
+  ///
+  /// The messages are processed by calling the given `callback`. Each message
+  /// is processed in its own microtask.
   Future<void> drain(String channel, DrainChannelCallback callback) async {
+    await null; // Ensures that the rest of this method is scheduled in a microtask.
     while (!_isEmpty(channel)) {
       final _StoredMessage message = _pop(channel)!;
       await callback(message.data, message.callback);

lib/web_ui/lib/src/ui/channel_buffers.dart

@@ -119,6 +119,7 @@ class ChannelBuffers {
   }
 
   Future<void> drain(String channel, DrainChannelCallback callback) async {
+    await null; // Ensures that the rest of this method is scheduled in a microtask.
     while (!_isEmpty(channel)) {
       final _StoredMessage message = _pop(channel)!;
       await callback(message.data, message.callback);

testing/dart/channel_buffers_test.dart

@@ -1,4 +1,6 @@
 // @dart = 2.6
+
+import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 import 'dart:ui' as ui;
@@ -30,6 +32,32 @@ void main() {
     });
   });
 
+  test('drain is async', () async {
+    const String channel = 'foo';
+    final ByteData data = _makeByteData('message');
+    final ui.ChannelBuffers buffers = ui.ChannelBuffers();
+    final ui.PlatformMessageResponseCallback callback = (ByteData responseData) {};
+    buffers.push(channel, data, callback);
+    final List<String> log = <String>[];
+    final Completer<void> completer = Completer<void>();
+    scheduleMicrotask(() { log.add('before drain, microtask'); });
+    log.add('before drain');
+    buffers.drain(channel, (ByteData drainedData, ui.PlatformMessageResponseCallback drainedCallback) async {
+      log.add('callback');
+      completer.complete();
+    });
+    log.add('after drain, before await');
+    await completer.future;
+    log.add('after await');
+    expect(log, <String>[
+      'before drain',
+      'after drain, before await',
+      'before drain, microtask',
+      'callback',
+      'after await'
+    ]);
+  });
+
   test('push drain zero', () async {
     const String channel = 'foo';
     final ByteData data = _makeByteData('bar');