Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

* Reland dispose images when done (#67100)

Changes since last time:

- Test for CanvasKit image rendering
  (https://github.com/flutter/flutter/pull/67176)
- Fix CanvasKit dispose impl
  (https://github.com/flutter/engine/pull/21555)
- Update internal google3 customer with a problematic ImageStream
  Listener impl (cl/335091311, cl/335459002)

This reverts commit 473358d9.

* Revert "docs for image disposal (#67066)"

This reverts commit bcb2ac5a.

* Revert "Dispose of images after using them (#66688)"

This reverts commit a7954691.

* Dispose of images after using them
Adds ImageInfo.clone, ImageInfo.dispose, and ImageInfo.isCloneOf.
Makes all stateful holders of ui.Image objects properly clone and dispose of them.
Behavior change for ImageStreamCompleter: Removing the last listener after registering at least one listener results in the object being disposed, and new listeners cannot be added, unless a keepAlive handle is created.
Makes all non-stateful holders of ui.Image (i.e. paintImage and RawImage) assert that whoever gave the image to them has not disposed it on them.

* Reland "Nnbd widgets (#64672)"

This reverts commit 127e6790.

* remove non-nullable enforcement on AsyncSnapshot

* make data param non-nullable for AsyncSnapshot.withData

* make some Text params non-nullable

This reverts commit e682ec71.

* migrate widget to nullsafety

* remove double blank line after license

* address review comments in actions.dart

* nullable ObjectKey.value

* use local variable oldElement

* make State.build non-nullable

* make State.context non-nullable

* newline at eof

* make ProxyWidget.child non-nullable

* make _InactiveElements.debugContains non-nullable

* make Element.depth non-nullable

* make ProxyElement.build non-nullable

* make StatefulElement.state non-nullable

* remove 'Notice that'

* avoid cast of list in RenderObjectElement.updateChildren

* make IndexedSlot.value non-nullable

* avoid cast of list in MultiChildRenderObjectElement.mount

* make some WidgetsApp parameters non-nullable

* hitTest take non-nullable position

* make ScrollableState.position non-nullable

* use _pixels instead of pixels

* make ViewportOffset.pixels non-nullable

* make param and return type of IndexedWidgetBuilder non-nullable

* unused_import

* make context param non-nullable for Builder in animated_list.dart

* make ScrollMetrics.viewportDimension non-nullable

* make ScrollMetrics.{min,max}ScrollExtent non-nullable

* make _Location.file non-nullable

* _WidgetForTypeTests.createElement throw UnimplementedError

* update _NullWidget.build error message

* make _ShortcutsState.manager non-nullable

* Fix childCount issues for NNBD

* fix childCount computation on web

* increase max value on js side to compute childCount

* make aspect parameter of dependOnInheritedWidgetOfExactType nullable

* merge has{min,max}ScrollExtent into hasScrollExtents

* update focus_manager.dart

* address review comments in icon.dart

* address review comments in image.dart

* address review comments in routes.dart

* address review comments in scroll_activity.dart

* update doc comments

* make UserScrollNotification.direction non-nullable and required

* rename hasScrollExtents to hasContentDimensions

* unnecessary late
Co-authored-by: Ian Hickson <ian@hixie.ch>

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Reland "Live image cache" (#51441)

This reverts commit 419a2853.

* Make evict safer