Merge pull request #783 from abarth/mimic_mark_needs_build

Expose markNeedsBuild on MimicOverlayEntry
diff --git a/packages/flutter/lib/src/widgets/mimic.dart b/packages/flutter/lib/src/widgets/mimic.dart
index acc1bde..f914d22 100644
--- a/packages/flutter/lib/src/widgets/mimic.dart
+++ b/packages/flutter/lib/src/widgets/mimic.dart
@@ -59,6 +59,10 @@
     return _performance.play();
   }
 
+  void markNeedsBuild() {
+   _overlayEntry?.markNeedsBuild();
+ }
+
   void dispose() {
     _targetKey = null;
     _curve = null;