Refactor platform message logic

This laids the groundwork for sending messages through ChannelBuffers with channel-specific callbacks rather than a single onPlatformMessage callback.

This allows us to remove the logic from the framework that puts data back into the channel buffers. Right now (before this PR) the logic for messages from plugins to the framework is bidirectional:

```
                  **                 *
Plugins -> Engine -> ChannelBuffers <- Framework <---+-.
               |              |                      | |
               |              '------> via drain ----' |
               |                                       |
               '----------------- onPlatformMessage ---'

* = when the listener is null on the framework side
** = when onPlatformMessage is null
```

This ends up with weird race conditions and is generally less than completely clear. With this PR, we lay the groundwork for eventually reaching this model:

```
Plugins -> Engine -> ChannelBuffers -> Framework
```

...which is significantly simpler.

Author: Hixie

lib/ui/channel_buffers.dart

@@ -159,6 +159,10 @@ void _invoke(void Function()? callback, Zone zone) {
 }
 
 /// Invokes [callback] inside the given [zone] passing it [arg].
+///
+/// The 1 in the name refers to the number of arguments expected by
+/// the callback (and thus passed to this function, in addition to the
+/// callback itself and the zone in which the callback is executed).
 void _invoke1<A>(void Function(A a)? callback, Zone zone, A arg) {
   if (callback == null) {
     return;
@@ -173,14 +177,33 @@ void _invoke1<A>(void Function(A a)? callback, Zone zone, A arg) {
   }
 }
 
+/// Invokes [callback] inside the given [zone] passing it [arg1] and [arg2].
+///
+/// The 2 in the name refers to the number of arguments expected by
+/// the callback (and thus passed to this function, in addition to the
+/// callback itself and the zone in which the callback is executed).
+void _invoke2<A1, A2>(void Function(A1 a1, A2 a2)? callback, Zone zone, A1 arg1, A2 arg2) {
+  if (callback == null) {
+    return;
+  }
+
+  assert(zone != null); // ignore: unnecessary_null_comparison
+
+  if (identical(zone, Zone.current)) {
+    callback(arg1, arg2);
+  } else {
+    zone.runGuarded(() {
+      callback(arg1, arg2);
+    });
+  }
+}
+
 /// Invokes [callback] inside the given [zone] passing it [arg1], [arg2], and [arg3].
-void _invoke3<A1, A2, A3>(
-  void Function(A1 a1, A2 a2, A3 a3)? callback,
-  Zone zone,
-  A1 arg1,
-  A2 arg2,
-  A3 arg3,
-) {
+///
+/// The 3 in the name refers to the number of arguments expected by
+/// the callback (and thus passed to this function, in addition to the
+/// callback itself and the zone in which the callback is executed).
+void _invoke3<A1, A2, A3>(void Function(A1 a1, A2 a2, A3 a3)? callback, Zone zone, A1 arg1, A2 arg2, A3 arg3) {
   if (callback == null) {
     return;
   }

lib/ui/hooks.dart

@@ -38,6 +38,7 @@ typedef SemanticsActionCallback = void Function(int id, SemanticsAction action,
 typedef PlatformMessageResponseCallback = void Function(ByteData? data);
 
 /// Signature for [PlatformDispatcher.onPlatformMessage].
+// TODO(ianh): deprecate once framework uses [ChannelBuffers.setListener].
 typedef PlatformMessageCallback = void Function(String name, ByteData? data, PlatformMessageResponseCallback? callback);
 
 // Signature for _setNeedsReportTimings.
@@ -413,6 +414,8 @@ class PlatformDispatcher {
   ///
   /// The framework invokes this callback in the same zone in which the callback
   /// was set.
+  // TODO(ianh): Deprecate onPlatformMessage once the framework is moved over
+  // to using channel buffers exclusively.
   PlatformMessageCallback? get onPlatformMessage => _onPlatformMessage;
   PlatformMessageCallback? _onPlatformMessage;
   Zone _onPlatformMessageZone = Zone.root;
@@ -442,13 +445,15 @@ class PlatformDispatcher {
     };
   }
 
-  // Called from the engine, via hooks.dart
+  /// Send a message to the framework using the [ChannelBuffers].
+  ///
+  /// This method constructs the appropriate callback to respond
+  /// with the given `responseId`. It should only be called for messages
+  /// from the platform.
   void _dispatchPlatformMessage(String name, ByteData? data, int responseId) {
     if (name == ChannelBuffers.kControlChannelName) {
       try {
         channelBuffers.handleMessage(data!);
-      } catch (ex) {
-        _printDebug('Message to "$name" caused exception $ex');
       } finally {
         _respondToPlatformMessage(responseId, null);
       }
@@ -458,7 +463,7 @@ class PlatformDispatcher {
         _onPlatformMessageZone,
         name,
         data,
-            (ByteData? responseData) {
+        (ByteData? responseData) {
           _respondToPlatformMessage(responseId, responseData);
         },
       );

lib/ui/platform_dispatcher.dart

@@ -2273,11 +2273,18 @@ final ByteData _fontChangeMessage = utf8.encoder.convert(
 ).buffer.asByteData();
 
 FutureOr<void> _sendFontChangeMessage() async {
-  PlatformDispatcher.instance.onPlatformMessage?.call(
-    'flutter/system',
-    _fontChangeMessage,
-    (_) {},
-  );
+  const String kSystemChannelName = 'flutter/system';
+  if (PlatformDispatcher.instance.onPlatformMessage != null) {
+    _invoke3<String, ByteData?, PlatformMessageResponseCallback>(
+      PlatformDispatcher.instance.onPlatformMessage,
+      PlatformDispatcher.instance._onPlatformMessageZone,
+      kSystemChannelName,
+      _fontChangeMessage,
+      (ByteData? responseData) { },
+    );
+  } else {
+    channelBuffers.push(kSystemChannelName, _fontChangeMessage, (ByteData? responseData) { });
+  }
 }
 
 String _loadFontFromList(Uint8List list, _Callback<void> callback, String? fontFamily) native 'loadFontFromList';

lib/ui/text.dart

@@ -696,6 +696,7 @@ class SingletonFlutterWindow extends FlutterWindow {
   ///
   /// The framework invokes this callback in the same zone in which the
   /// callback was set.
+  // TODO(ianh): deprecate once framework uses [ChannelBuffers.setListener].
   PlatformMessageCallback? get onPlatformMessage => platformDispatcher.onPlatformMessage;
   set onPlatformMessage(PlatformMessageCallback? callback) {
     platformDispatcher.onPlatformMessage = callback;

lib/ui/window.dart

@@ -81,10 +81,6 @@ class Keyboard {
 
     final html.KeyboardEvent keyboardEvent = event;
 
-    if (EnginePlatformDispatcher.instance._onPlatformMessage == null) {
-      return;
-    }
-
     if (_shouldPreventDefault(event)) {
       event.preventDefault();
     }

lib/web_ui/lib/src/engine/keyboard.dart

@@ -151,17 +151,15 @@ class MultiEntriesBrowserHistory extends BrowserHistory {
           currentPath);
     }
     _lastSeenSerialCount = _currentSerialCount;
-    if (EnginePlatformDispatcher.instance._onPlatformMessage != null) {
-      EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
-        'flutter/navigation',
-        const JSONMethodCodec().encodeMethodCall(
-            MethodCall('pushRouteInformation', <dynamic, dynamic>{
-          'location': currentPath,
-          'state': event.state?['state'],
-        })),
-        (_) {},
-      );
-    }
+    EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
+      'flutter/navigation',
+      const JSONMethodCodec().encodeMethodCall(
+          MethodCall('pushRouteInformation', <dynamic, dynamic>{
+        'location': currentPath,
+        'state': event.state?['state'],
+      })),
+      (_) {},
+    );
   }
 
   @override
@@ -272,13 +270,11 @@ class SingleEntryBrowserHistory extends BrowserHistory {
       _setupFlutterEntry(urlStrategy!);
 
       // 2. Send a 'popRoute' platform message so the app can handle it accordingly.
-      if (EnginePlatformDispatcher.instance._onPlatformMessage != null) {
-        EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
-          'flutter/navigation',
-          const JSONMethodCodec().encodeMethodCall(_popRouteMethodCall),
-          (_) {},
-        );
-      }
+      EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
+        'flutter/navigation',
+        const JSONMethodCodec().encodeMethodCall(_popRouteMethodCall),
+        (_) {},
+      );
     } else if (_isFlutterEntry(event.state)) {
       // We get into this scenario when the user changes the url manually. It
       // causes a new entry to be pushed on top of our "flutter" one. When this
@@ -291,15 +287,13 @@ class SingleEntryBrowserHistory extends BrowserHistory {
       _userProvidedRouteName = null;
 
       // Send a 'pushRoute' platform message so the app handles it accordingly.
-      if (EnginePlatformDispatcher.instance._onPlatformMessage != null) {
-        EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
-          'flutter/navigation',
-          const JSONMethodCodec().encodeMethodCall(
-            MethodCall('pushRoute', newRouteName),
-          ),
-          (_) {},
-        );
-      }
+      EnginePlatformDispatcher.instance.invokeOnPlatformMessage(
+        'flutter/navigation',
+        const JSONMethodCodec().encodeMethodCall(
+          MethodCall('pushRoute', newRouteName),
+        ),
+        (_) {},
+      );
     } else {
       // The user has pushed a new entry on top of our flutter entry. This could
       // happen when the user modifies the hash part of the url directly, for