Merge pull request #2453 from Hixie/placeholder-assert

Clarify the assert when mimicking twice in a row.

packages/flutter/lib/src/widgets/mimic.dart

@@ -176,7 +176,18 @@ class MimicableState extends State<Mimicable> {
   /// passing it to a [Mimic] widget. To mimic the child in the
   /// [Overlay], consider using [liftToOverlay()] instead.
   MimicableHandle startMimic() {
-    assert(_placeholderSize == null);
+    assert(() {
+      if (_placeholderSize != null) {
+        throw new WidgetError(
+          'Mimicable started while already active.\n'
+          'When startMimic() or liftToOverlay() is called on a MimicableState, the mimic becomes active. '
+          'While active, it cannot be reactivated until it is stopped. '
+          'To stop a Mimicable started with startMimic(), call the MimicableHandle object\'s stopMimic() method. '
+          'To stop a Mimicable started with liftToOverlay(), call dispose() on the MimicOverlayEntry.'
+        );
+      }
+      return true;
+    });
     RenderBox box = context.findRenderObject();
     assert(box != null);
     assert(box.hasSize);