[Material] Fix 20051 FAB tooltip touch target (#21084)

* [FAB] Updated tooltip touch target.

* Remove "new" keyword

* [FAB] Updated tooltip touch target.

* "long press button edge"

* remove new

* remove new

* put "new" keywords back in

* Remove check for childless tooltip

* Added regression test - tooltip works on edge of FAB which has no child.

Added helper method to find the right edge of a in the tests.

* Changed "find.byType(text)" to "find.text('Add')"

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

diff --git a/packages/flutter/lib/src/material/floating_action_button.dart b/packages/flutter/lib/src/material/floating_action_button.dart
index 5a43297..48be7dc 100644
--- a/packages/flutter/lib/src/material/floating_action_button.dart
+++ b/packages/flutter/lib/src/material/floating_action_button.dart
@@ -249,16 +249,6 @@
       );
     }
 
-    if (widget.tooltip != null) {
-      final Widget tooltip = new Tooltip(
-        message: widget.tooltip,
-        child: result,
-      );
-      // The long-pressable area for the tooltip should always be as big as
-      // the tooltip even if there is no child.
-      result = widget.child != null ? tooltip : new SizedBox.expand(child: tooltip);
-    }
-
     result = new RawMaterialButton(
       onPressed: widget.onPressed,
       onHighlightChanged: _handleHighlightChanged,
@@ -275,6 +265,15 @@
       child: result,
     );
 
+    if (widget.tooltip != null) {
+      result = new MergeSemantics(
+        child: new Tooltip(
+          message: widget.tooltip,
+          child: result,
+        ),
+      );
+    }
+
     if (widget.heroTag != null) {
       result = new Hero(
         tag: widget.heroTag,

packages/flutter/test/material/floating_action_button_test.dart

diff --git a/packages/flutter/test/material/floating_action_button_test.dart b/packages/flutter/test/material/floating_action_button_test.dart
index a79892b..09aa3fd 100644
--- a/packages/flutter/test/material/floating_action_button_test.dart
+++ b/packages/flutter/test/material/floating_action_button_test.dart
@@ -51,6 +51,47 @@
     expect(find.byTooltip('Add'), findsOneWidget);
   });
 
+  // Regression test for: https://github.com/flutter/flutter/pull/21084
+  testWidgets('Floating Action Button tooltip (long press button edge)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: null,
+            tooltip: 'Add',
+            child: Icon(Icons.add),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('Add'), findsNothing);
+    await tester.longPressAt(_rightEdgeOfFab(tester));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Add'), findsOneWidget);
+  });
+
+  // Regression test for: https://github.com/flutter/flutter/pull/21084
+  testWidgets('Floating Action Button tooltip (long press button edge - no child)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: null,
+            tooltip: 'Add',
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('Add'), findsNothing);
+    await tester.longPressAt(_rightEdgeOfFab(tester));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Add'), findsOneWidget);
+  });
+
   testWidgets('Floating Action Button tooltip (no child)', (WidgetTester tester) async {
     await tester.pumpWidget(
       new MaterialApp(
@@ -63,10 +104,10 @@
       ),
     );
 
-    expect(find.byType(Text), findsNothing);
+    expect(find.text('Add'), findsNothing);
     await tester.longPress(find.byType(FloatingActionButton));
     await tester.pumpAndSettle();
-    expect(find.byType(Text), findsOneWidget);
+    expect(find.text('Add'), findsOneWidget);
   });
 
   testWidgets('FlatActionButton mini size is configurable by ThemeData.materialTapTargetSize', (WidgetTester tester) async {
@@ -458,3 +499,8 @@
     );
   });
 }
+
+Offset _rightEdgeOfFab(WidgetTester tester) {
+  final Finder fab = find.byType(FloatingActionButton);
+  return tester.getRect(fab).centerRight - const Offset(1.0, 0.0);
+}