apply null safety syntax to mobile dart:ui (flutter#18933)

* apply null safety syntax to mobile dart:ui

analysis_options.yaml

@@ -10,6 +10,8 @@
 #    private fields, especially on the Window object):
 
 analyzer:
+  enable-experiment:
+    - non-nullable
   exclude: [
     # this test pretends to be part of dart:ui and results in lots of false
     # positives.

ci/analyze.sh

@@ -7,6 +7,7 @@ dartanalyzer --version
 
 RESULTS=`dartanalyzer                                                          \
   --options flutter/analysis_options.yaml                                      \
+  --enable-experiment=non-nullable                                             \
   "$1out/host_debug_unopt/gen/sky/bindings/dart_ui/ui.dart"                    \
   2>&1                                                                         \
   | grep -Ev "No issues found!"                                                \

lib/ui/annotations.dart

@@ -4,7 +4,8 @@
 
 // TODO(dnfield): Remove unused_import ignores when https://github.com/dart-lang/sdk/issues/35164 is resolved.
 
-// @dart = 2.6
+// @dart = 2.9
+
 part of dart.ui;
 
 // TODO(dnfield): Update this if/when we default this to on in the tool,
@@ -39,7 +40,7 @@ part of dart.ui;
 ///   }
 /// }
 /// ```
-const _KeepToString/*!*/ keepToString = _KeepToString();
+const _KeepToString keepToString = _KeepToString();
 
 class _KeepToString {
   const _KeepToString();

lib/ui/channel_buffers.dart

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.6
+// @dart = 2.9
+
 part of dart.ui;
 
 /// A saved platform message for a channel with its callback.
@@ -14,12 +15,12 @@ class _StoredMessage {
   _StoredMessage(this._data, this._callback);
 
   /// Representation of the message's payload.
-  final ByteData/*?*/ _data;
-  ByteData/*?*/ get data => _data;
+  final ByteData? _data;
+  ByteData? get data => _data;
 
   /// Callback to be called when the message is received.
-  final PlatformMessageResponseCallback/*!*/ _callback;
-  PlatformMessageResponseCallback/*!*/ get callback => _callback;
+  final PlatformMessageResponseCallback _callback;
+  PlatformMessageResponseCallback get callback => _callback;
 }
 
 /// A fixed-size circular queue.
@@ -43,7 +44,7 @@ class _RingBuffer<T> {
 
   /// A callback that get's called when items are ejected from the [_RingBuffer]
   /// by way of an overflow or a resizing.
-  Function(T) _dropItemCallback;
+  Function(T)? _dropItemCallback;
   set dropItemCallback(Function(T) callback) {
     _dropItemCallback = callback;
   }
@@ -60,7 +61,7 @@ class _RingBuffer<T> {
   }
 
   /// Returns null when empty.
-  T pop() {
+  T? pop() {
     return _queue.isEmpty ? null : _queue.removeFirst();
   }
 
@@ -70,9 +71,7 @@ class _RingBuffer<T> {
     int result = 0;
     while (_queue.length > lengthLimit) {
       final T item = _queue.removeFirst();
-      if (_dropItemCallback != null) {
-        _dropItemCallback(item);
-      }
+      _dropItemCallback?.call(item);
       result += 1;
     }
     return result;
@@ -86,7 +85,7 @@ class _RingBuffer<T> {
 }
 
 /// Signature for [ChannelBuffers.drain].
-typedef DrainChannelCallback = Future<void>/*!*/ Function(ByteData/*?*/, PlatformMessageResponseCallback/*!*/);
+typedef DrainChannelCallback = Future<void> Function(ByteData?, PlatformMessageResponseCallback);
 
 /// Storage of channel messages until the channels are completely routed,
 /// i.e. when a message handler is attached to the channel on the framework side.
@@ -116,8 +115,8 @@ class ChannelBuffers {
   static const String kControlChannelName = 'dev.flutter/channel-buffers';
 
   /// A mapping between a channel name and its associated [_RingBuffer].
-  final Map<String, _RingBuffer<_StoredMessage>> _messages =
-    <String, _RingBuffer<_StoredMessage>>{};
+  final Map<String, _RingBuffer<_StoredMessage>?> _messages =
+    <String, _RingBuffer<_StoredMessage>?>{};
 
   _RingBuffer<_StoredMessage> _makeRingBuffer(int size) {
     final _RingBuffer<_StoredMessage> result = _RingBuffer<_StoredMessage>(size);
@@ -130,8 +129,8 @@ class ChannelBuffers {
   }
 
   /// Returns true on overflow.
-  bool/*!*/ push(String/*!*/ channel, ByteData/*?*/ data, PlatformMessageResponseCallback/*!*/ callback) {
-    _RingBuffer<_StoredMessage> queue = _messages[channel];
+  bool push(String channel, ByteData? data, PlatformMessageResponseCallback callback) {
+    _RingBuffer<_StoredMessage>? queue = _messages[channel];
     if (queue == null) {
       queue = _makeRingBuffer(kDefaultBufferSize);
       _messages[channel] = queue;
@@ -150,23 +149,23 @@ class ChannelBuffers {
   }
 
   /// Returns null on underflow.
-  _StoredMessage _pop(String channel) {
-    final _RingBuffer<_StoredMessage> queue = _messages[channel];
-    final _StoredMessage result = queue?.pop();
+  _StoredMessage? _pop(String channel) {
+    final _RingBuffer<_StoredMessage>? queue = _messages[channel];
+    final _StoredMessage? result = queue?.pop();
     return result;
   }
 
   bool _isEmpty(String channel) {
-    final _RingBuffer<_StoredMessage> queue = _messages[channel];
-    return (queue == null) ? true : queue.isEmpty;
+    final _RingBuffer<_StoredMessage>? queue = _messages[channel];
+    return queue == null || queue.isEmpty;
   }
 
   /// Changes the capacity of the queue associated with the given channel.
   ///
   /// This could result in the dropping of messages if newSize is less
   /// than the current length of the queue.
   void _resize(String channel, int newSize) {
-    _RingBuffer<_StoredMessage> queue = _messages[channel];
+    _RingBuffer<_StoredMessage>? queue = _messages[channel];
     if (queue == null) {
       queue = _makeRingBuffer(newSize);
       _messages[channel] = queue;
@@ -182,9 +181,9 @@ 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).
-  Future<void>/*!*/ drain(String/*!*/ channel, DrainChannelCallback/*!*/ callback) async {
+  Future<void> drain(String channel, DrainChannelCallback callback) async {
     while (!_isEmpty(channel)) {
-      final _StoredMessage message = _pop(channel);
+      final _StoredMessage message = _pop(channel)!;
       await callback(message.data, message.callback);
     }
   }
@@ -204,7 +203,7 @@ class ChannelBuffers {
   ///   Arity: 2
   ///   Format: `resize\r<channel name>\r<new size>`
   ///   Description: Allows you to set the size of a channel's buffer.
-  void handleMessage(ByteData/*!*/ data) {
+  void handleMessage(ByteData data) {
     final List<String> command = _getString(data).split('\r');
     if (command.length == /*arity=*/2 + 1 && command[0] == 'resize') {
       _resize(command[1], int.parse(command[2]));
@@ -220,4 +219,4 @@ class ChannelBuffers {
 ///
 /// See also:
 /// * [BinaryMessenger] - The place where ChannelBuffers are typically read.
-final ChannelBuffers/*!*/ channelBuffers = ChannelBuffers();
+final ChannelBuffers channelBuffers = ChannelBuffers();