Issue 1:
Switching between the different app bar display options in the "Contact profile" study can cause the Flutter Gallery app to crash. I tested this on a OnePlus3T using the production version on the PlayStore and locally using the version in v0.5.1.

@HansMuller I think this is behind the error in #17598

Steps to Reproduce

Open the "Contact profile" study

Scroll the bottom of the page

Select the drop down menu in top right of the app bar and choose the "App bar snaps" option

Scroll up until the full app bar is displayed (screenshot attached)

Select the "App bar floats" option

The app will not immediately crash but it is in a bad state and further actions will cause various exceptions and crashes. Logs attached below.

Logs

Error when scrolling:

[+34049 ms] I/flutter (29797): ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
[ +19 ms] I/flutter (29797): The following NoSuchMethodError was thrown while dispatching notifications for ValueNotifier<bool>:
[ ] I/flutter (29797): The method 'stop' was called on null.
[ ] I/flutter (29797): Receiver: null
[ +1 ms] I/flutter (29797): Tried calling: stop(canceled: true)
[ ] I/flutter (29797):
[ ] I/flutter (29797): When the exception was thrown, this was the stack:
[ +12 ms] I/flutter (29797): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
[ ] I/flutter (29797): #1 AnimationController.stop (package:flutter/src/animation/animation_controller.dart:499:13)
[ ] I/flutter (29797): #2 RenderSliverFloatingPersistentHeader.maybeStopSnapAnimation (package:flutter/src/rendering/sliver_persistent_header.dart:457:18)
[ ] I/flutter (29797): #3 _FloatingAppBarState._isScrollingListener (package:flutter/src/material/app_bar.dart:547:15)
[ ] I/flutter (29797): #4 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:161:21)
[ ] I/flutter (29797): #5 ValueNotifier.value= (package:flutter/src/foundation/change_notifier.dart:217:5)
[ ] I/flutter (29797): #6 ScrollPosition.beginActivity (package:flutter/src/widgets/scroll_position.dart:613:25)
[ ] I/flutter (29797): #7 ScrollPositionWithSingleContext.beginActivity (package:flutter/src/widgets/scroll_position_with_single_context.dart:117:11)
[ ] I/flutter (29797): #8 ScrollPositionWithSingleContext.drag (package:flutter/src/widgets/scroll_position_with_single_context.dart:245:5)
[ ] I/flutter (29797): #9 ScrollableState._handleDragStart (package:flutter/src/widgets/scrollable.dart:443:22)
[ +1 ms] I/flutter (29797): #10 DragGestureRecognizer.acceptGesture.<anonymous closure> (package:flutter/src/gestures/monodrag.dart:169:47)
[ ] I/flutter (29797): #11 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
[ ] I/flutter (29797): #12 DragGestureRecognizer.acceptGesture (package:flutter/src/gestures/monodrag.dart:169:9)
[ ] I/flutter (29797): #13 GestureArenaManager._resolveByDefault (package:flutter/src/gestures/arena.dart:250:25)
[ ] I/flutter (29797): #14 GestureArenaManager._tryToResolveArena.<anonymous closure> (package:flutter/src/gestures/arena.dart:231:31)
[ ] I/flutter (29797): (elided 2 frames from package dart:async)
[ ] I/flutter (29797):
[ ] I/flutter (29797): The ValueNotifier<bool> sending notification was:
[ ] I/flutter (29797): ValueNotifier<bool>#0a1d4(true)
[ ] I/flutter (29797): ════════════════════════════════════════════════════════════════════════════════════════════════════

Error when trying to switch to the 'App bar snaps' option:

[+40490 ms] I/flutter (29797): Another exception was thrown: AnimationController.dispose() called more than once.
[ +8 ms] I/flutter (29797): Another exception was thrown: A RenderViewport expected a child of type RenderSliver but received a child of type RenderErrorBox.
[ +9 ms] I/chatty (29797): uid=10084(io.flutter.demo.gallery) 1.ui identical 1 line
[ ] I/flutter (29797): Another exception was thrown: A RenderViewport expected a child of type RenderSliver but received a child of type RenderErrorBox.
[ ] I/flutter (29797): Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 3497 pos 14: 'owner._debugCurrentBuildTarget == this': is not true.
[+2095 ms] I/chatty (29797): uid=10084(io.flutter.demo.gallery) 1.ui identical 20 lines
[ ] I/flutter (29797): Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 3497 pos 14: 'owner._debugCurrentBuildTarget == this': is not true.

Error when clicking the back button

[+242379 ms] I/flutter (29797): Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4062 pos 14: '() {
[ +279 ms] I/flutter (29797): Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 2240 pos 16: '!_dirtyElements[index]._active || _dirtyElements[index]._debugIsInScope(context)': is not true.

Error when trying to re-open the Contact profile study after the last crash

[+457845 ms] I/flutter (29797): Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 3681 pos 12: 'child == _child': is not true.
[ +6 ms] I/flutter (29797): Another exception was thrown: Duplicate GlobalKey detected in widget tree.

Issue 2:
Flutter doctor is amazing! But, for the first hour, it may be (incorrectly) suggesting that you need both iOS and Android toolchains installed just to try Flutter.

See anecdote here: https://twitter.com/clarklab/status/998613583599669250