Wrap input widgets in a repaint boundary (#6248)

Performance improvement for https://github.com/flutter/flutter/issues/6008

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

@@ -255,13 +255,15 @@ class _InputState extends State<Input> {
       );
     }
 
-    return new GestureDetector(
+    return new RepaintBoundary(
+      child: new GestureDetector(
         behavior: HitTestBehavior.opaque,
         onTap: () => _rawInputLineKey.currentState?.requestKeyboard(),
         child: new Padding(
           padding: const EdgeInsets.symmetric(horizontal: 16.0),
           child: child
         )
+      )
     );
   }
 }