Add SemanticsEvent.toString for easier debugging (#12363)

packages/flutter/lib/src/rendering/semantics_event.dart

@@ -26,6 +26,16 @@ abstract class SemanticsEvent {
   /// Converts this event to a Map that can be encoded with
   /// [StandardMessageCodec].
   Map<String, dynamic> toMap();
+
+  @override
+  String toString() {
+    final List<String> pairs = <String>[];
+    final Map<String, dynamic> map = toMap();
+    final List<String> sortedKeys = map.keys.toList()..sort();
+    for (String key in sortedKeys)
+      pairs.add('$key: ${map[key]}');
+    return '$runtimeType(${pairs.join(', ')})';
+  }
 }
 
 /// Notifies that a scroll action has been completed.

packages/flutter/test/widgets/semantics_event_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('SemanticsEvent.toString', () {
+    expect(
+      new TestSemanticsEvent().toString(),
+      'TestSemanticsEvent()',
+    );
+    expect(
+      new TestSemanticsEvent(number: 10).toString(),
+      'TestSemanticsEvent(number: 10)',
+    );
+    expect(
+      new TestSemanticsEvent(text: 'hello').toString(),
+      'TestSemanticsEvent(text: hello)',
+    );
+    expect(
+      new TestSemanticsEvent(text: 'hello', number: 10).toString(),
+      'TestSemanticsEvent(number: 10, text: hello)',
+    );
+  });
+}
+
+class TestSemanticsEvent extends SemanticsEvent {
+  TestSemanticsEvent({ this.text, this.number }) : super('TestEvent');
+
+  final String text;
+  final int number;
+
+  @override
+  Map<String, dynamic> toMap() {
+    final Map<String, dynamic> result = <String, dynamic>{};
+    if (text != null)
+      result['text'] = text;
+    if (number != null)
+      result['number'] = number;
+    return result;
+  }
+}