Stocks demo tap fails if BottomSheet is visible #382
Description
Currently if you tap on a stock row (to bring up the StockSymbolPage for that stock) when the BottomSheet is visible there's an assert:
android: I/sky : -- EXCEPTION IN SCHEDULER CALLBACK --
android: I/sky : 'packages/flutter/src/widgets/heroes.dart': Failed assertion: line 125: '!heroes[tag].containsKey(null)' is not true.
android: I/sky : Stack trace:
android: I/sky : #0 _AssertionError._throwNew (dart:core-patch/errors_patch.dart:27)
android: I/sky : #1 Hero.of (packages/flutter/src/widgets/heroes.dart:125)
android: I/sky : #2 HeroController._updateQuest (packages/flutter/src/widgets/hero_controller.dart:126)
android: I/sky : #3 Scheduler.invokeCallback (packages/flutter/src/animation/scheduler.dart:81)
android: I/sky : #4 Scheduler.beginFrame (packages/flutter/src/animation/scheduler.dart:73)
android: I/sky : #5 _beginFrame (file:///usr/local/google/home/johnmccutchan/workspace/flutter/engine/src/out/android_Release/gen/sky/bindings/Customhooks.dart:35)