Fix FAB Tooltip semantics label (#16072)

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

@@ -253,6 +253,17 @@ class _FloatingActionButtonState extends State<FloatingActionButton> {
       );
     }
 
+    if (widget.tooltip != null) {
+      // The long-pressable area for the tooltip should always be as big as
+      // the tooltip even if there is no child.
+      result = new SizedBox.expand(
+        child: new Tooltip(
+          message: widget.tooltip,
+          child: result,
+        ),
+      );
+    }
+
     result = new RawMaterialButton(
       onPressed: widget.onPressed,
       onHighlightChanged: _handleHighlightChanged,
@@ -267,13 +278,6 @@ class _FloatingActionButtonState extends State<FloatingActionButton> {
       child: result,
     );
 
-    if (widget.tooltip != null) {
-      result = 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

@@ -253,6 +253,40 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('Tooltip is used as semantics label', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          floatingActionButton: new FloatingActionButton(
+            onPressed: () { },
+            tooltip: 'Add Photo',
+            child: const Icon(Icons.add_a_photo),
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          label: 'Add Photo',
+          actions: <SemanticsAction>[
+            SemanticsAction.tap
+          ],
+          flags: <SemanticsFlag>[
+            SemanticsFlag.isButton,
+            SemanticsFlag.hasEnabledState,
+            SemanticsFlag.isEnabled,
+          ],
+        ),
+      ],
+    ), ignoreTransform: true, ignoreId: true, ignoreRect: true));
+
+    semantics.dispose();
+  });
+
   group('ComputeNotch', () {
     testWidgets('host and guest must intersect', (WidgetTester tester) async {
       final ComputeNotch computeNotch = await fetchComputeNotch(tester, const FloatingActionButton(onPressed: null));