Add tooltips to FloatingActionButtons

Fixes #1875

examples/material_gallery/lib/demo/buttons_demo.dart

@@ -77,6 +77,7 @@ class _ButtonsDemoState extends State<ButtonsDemo> {
       height: 128.0,
       child: new Center(
         child: new FloatingActionButton(
+          tooltip: 'Open FAB demos',
           child: new Icon(icon: 'content/add'),
           onPressed: () {
             Navigator.push(context, new MaterialPageRoute(

examples/material_gallery/lib/demo/tabs_fab_demo.dart

@@ -102,6 +102,7 @@ class _TabsFabDemoState extends State<TabsFabDemo> {
         ),
         floatingActionButton: !selectedPage.fabDefined ? null : new FloatingActionButton(
           key: selectedPage.fabKey,
+          tooltip: 'Show explanation',
           backgroundColor: selectedPage.fabColor,
           child: selectedPage.fabIcon,
           onPressed: _showExplanatoryText

examples/stocks/lib/stock_home.dart

@@ -272,6 +272,7 @@ class StockHomeState extends State<StockHome> {
 
   Widget buildFloatingActionButton() {
     return new FloatingActionButton(
+      tooltip: 'Create company',
       child: new Icon(icon: 'content/add'),
       backgroundColor: Colors.redAccent[200],
       onPressed: _handleCreateCompany

examples/widgets/http_post.dart

@@ -62,6 +62,7 @@ class PostDemoState extends State<PostDemo> {
         )
       ),
       floatingActionButton: new FloatingActionButton(
+        tooltip: 'Refresh',
         child: new Icon(
           icon: 'navigation/refresh'
         ),