Add SemanticService.tooltip method for Android tooltips (#16978)

packages/flutter/lib/src/material/tooltip.dart

@@ -147,6 +147,7 @@ class _TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
     _entry = new OverlayEntry(builder: (BuildContext context) => overlay);
     Overlay.of(context, debugRequiredFor: widget).insert(_entry);
     GestureBinding.instance.pointerRouter.addGlobalRoute(_handlePointerEvent);
+    SemanticsService.tooltip(widget.message);
     _controller.forward();
     return true;
   }

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

@@ -88,3 +88,22 @@ class AnnounceSemanticsEvent extends SemanticsEvent {
     };
   }
 }
+
+/// An event for a semantic announcement of a tooltip.
+/// 
+/// This is only used by Android to announce tooltip values.
+class TooltipSemanticsEvent extends SemanticsEvent {
+
+  /// Constructs an event that triggers a tooltip announcement by the platform.
+  const TooltipSemanticsEvent(this.message) : super('tooltip');
+
+  /// The text content of the tooltip.
+  final String message;
+
+  @override
+  Map<String, dynamic> getDataMap() {
+    return <String, dynamic>{
+      'message': message,
+    };
+  }
+}

packages/flutter/lib/src/semantics/semantics_service.dart

@@ -7,7 +7,7 @@ import 'dart:ui' show TextDirection;
 
 import 'package:flutter/services.dart' show SystemChannels;
 
-import 'semantics_event.dart' show AnnounceSemanticsEvent;
+import 'semantics_event.dart' show AnnounceSemanticsEvent, TooltipSemanticsEvent;
 
 
 /// Allows access to the platform's accessibility services.
@@ -31,4 +31,12 @@ class SemanticsService {
     final AnnounceSemanticsEvent event = new AnnounceSemanticsEvent(message, textDirection);
     await SystemChannels.accessibility.send(event.toMap());
   }
+
+  /// Sends a semantic announcement of a tooltip.
+  /// 
+  /// This is only used by Android.
+  static Future<Null> tooltip(String message) async {
+    final TooltipSemanticsEvent event = new TooltipSemanticsEvent(message);
+    await SystemChannels.accessibility.send(event.toMap());
+  }
 }