fix analyzer warning: use const where possible (#9320)

dev/benchmarks/complex_layout/lib/main.dart

@@ -482,11 +482,11 @@ class ItemGalleryBox extends StatelessWidget {
                             new Row(
                               children: <Widget>[
                                 new IconButton(
-                                  icon: new Icon(Icons.share),
+                                  icon: const Icon(Icons.share),
                                   onPressed: () { print('Pressed share'); },
                                 ),
                                 new IconButton(
-                                  icon: new Icon(Icons.event),
+                                  icon: const Icon(Icons.event),
                                   onPressed: () { print('Pressed event'); },
                                 ),
                                 new Expanded(
@@ -620,7 +620,7 @@ class FancyDrawerHeader extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return new Container(
-      decoration: new BoxDecoration(
+      decoration: const BoxDecoration(
         backgroundColor: Colors.purple
       ),
       height: 200.0

dev/manual_tests/card_collection.dart

@@ -286,7 +286,7 @@ class CardCollectionState extends State<CardCollection> {
       leftArrowIcon = new Opacity(opacity: 0.1, child: leftArrowIcon);
 
       // TODO(abarth): This icon is wrong in RTL.
-    Widget rightArrowIcon =  new Icon(Icons.arrow_forward, size: 36.0);
+    Widget rightArrowIcon =  const Icon(Icons.arrow_forward, size: 36.0);
     if (_dismissDirection == DismissDirection.endToStart)
       rightArrowIcon = new Opacity(opacity: 0.1, child: rightArrowIcon);
 

dev/manual_tests/page_view.dart

@@ -86,7 +86,7 @@ class PageViewAppState extends State<PageViewApp> {
         children: <Widget>[
           const DrawerHeader(child: const Center(child: const Text('Options'))),
           new ListTile(
-            leading: new Icon(Icons.more_horiz),
+            leading: const Icon(Icons.more_horiz),
             selected: scrollDirection == Axis.horizontal,
             trailing: const Text('Horizontal Layout'),
             onTap: switchScrollDirection,

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

@@ -378,7 +378,7 @@ class _AppBarState extends State<AppBar> {
     if (leading == null) {
       if (_hasDrawer) {
         leading = new IconButton(
-          icon: new Icon(Icons.menu),
+          icon: const Icon(Icons.menu),
           onPressed: _handleDrawerButton,
           tooltip: 'Open navigation menu' // TODO(ianh): Figure out how to localize this string
         );

packages/flutter_tools/lib/src/version.dart

@@ -294,7 +294,7 @@ class VersionCheckStamp {
     }
 
     // Stamp is missing or is malformed.
-    return new VersionCheckStamp();
+    return const VersionCheckStamp();
   }
 
   static VersionCheckStamp fromJson(Map<String, String> json) {