linter: Convert Flutter utilities into extension getters and methods

There were some surprising wins in here that I'm happy with. They can
be seen in the call-sites in the various lint rules below.

There were two top-level functions (`isWidgetTypeContainer`, `isWidgetTypeSizedBox`) that _were_ concerned with the `staticType` of
an InstanceCreationExpression. This isn't a coincidence: when a
developer is writing Flutter code, they _never_ mess with an
instantiated Widget; all they do is _instantiate_ a Widget. So we can
conveniently just make these extension getters on
InstanceCreationExpression instead, and make the getters more friendly.

Several of these top-level functions answer a question about a
`DartType?` object. (`isWidgetProperty`, `isWidgetType`,
`isBuildContext`.) Each of these are moved to be an extension getter or
method on `DartType?`.

`isKDebugMode` answers a question about an `Element?`, so it is made an
extension getter.

`isState` and `isStatefulWidget` answer questions about
`InterfaceElement`s, and can each be moved to be extension getters.

Change-Id: Ibe45095c612363f470b96719871ed36728f62e66
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/471886
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>