Nested Navigators don't work correctly. #344
Description
In this example I have an outer navigator (in the FlutterApp) and an inner one (as a child of the scaffold).
If you press the "next" button instead of seeing the /1 page (which says "one") you still see the / "hello world" page. Then if you press back you get a stack:
android: I/sky : ------------------------------------------------------------------------
android: I/sky : Exception caught while building Focus-[GlobalObjectKey PageRoute(367453558)]
android: I/sky : 'packages/flutter/src/widgets/framework.dart': Failed assertion: line 133: 'added' 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 GlobalKey.registerRemoveListener (packages/flutter/src/widgets/framework.dart:133)
android: I/sky : #2 FocusState._updateScopeRemovalListener (packages/flutter/src/widgets/focus.dart:206)
android: I/sky : #3 FocusState._setFocusedScopeIfUnset (packages/flutter/src/widgets/focus.dart:189)
android: I/sky : #4 _FocusScope._setFocusedScopeIfUnset (packages/flutter/src/widgets/focus.dart:45)
android: I/sky : #5 Focus._atScope (packages/flutter/src/widgets/focus.dart:106)
android: I/sky : #6 FocusState.build (packages/flutter/src/widgets/focus.dart:226)
android: I/sky : #7 ComponentElement.performRebuild (packages/flutter/src/widgets/framework.dart:1050)
android: I/sky : #8 BuildableElement.rebuild (packages/flutter/src/widgets/framework.dart:991)
android: I/sky : #9 WidgetFlutterBinding.buildDirtyElements.<anonymous closure> (packages/flutter/src/widgets/binding.dart:55)
android: I/sky : #10 BuildableElement.lockState (packages/flutter/src/widgets/framework.dart:924)
android: I/sky : #11 WidgetFlutterBinding.buildDirtyElements (packages/flutter/src/widgets/binding.dart:50)
android: I/sky : #12 WidgetFlutterBinding.beginFrame (packages/flutter/src/widgets/binding.dart:26)
android: I/sky : #13 FlutterBinding._handlePersistentFrameCallback (packages/flutter/src/rendering/binding.dart:176)
android: I/sky : #14 Scheduler.invokeCallback (packages/flutter/src/animation/scheduler.dart:81)
android: I/sky : #15 Scheduler.beginFrame (packages/flutter/src/animation/scheduler.dart:67)
android: I/sky : #16 _beginFrame (file:///src/flutter/engine/src/out/android_Release/gen/sky/bindings/Customhooks.dart:30)
android: I/sky : ------------------------------------------------------------------------
[main-dart.txt](https://github.com/flutter/flutter/files/33382/main-dart.txt)