* Add onCancelled callback to PopupMenu

* Fix spelling, don't call onCanceled if disposed, improve documentation.

* Remove MediaQuery padding for various PopupRoutes that don't extend to the edge of the screen

* bottom sheet doesn't actually need handling. Scaffold does it already. Add test for others.

This fixes the popup menu code to do a better job of expanding
smoothly regardless of which side of the screen it's on. It still
results in a bidirection growth when positioned at the bottom of the
screen, so maybe we'll need to animate menus differently, but that's
a problem for another patch.

Also, improve some docs and provide RelativeRect.toSize which I needed
at one point while building this patch (though it didn't survive all
the way to the end).

* upgrade to linter-0.1.35

* re-enable lint prefer_const_constructors

* address review comments

* PopupMenuButton: create IconButton if child is Icon

Otherwise the resulting button has an abnormally small and rectangular
area. With multiple PopupMenuButton(child: Icon) they get squished
together in the AppBar.

* Add separate icon argument to PopupMenuButton

* Fix style issues and tweak dartdocs

* Add tests for icon argument to PopupMenuButton

* Group icon tests and fix broken test, analyzer warnings

* Test that the correct custom icon is present

* Apply De Morgan's to work around dart analyzer bug

see: https://github.com/dart-lang/sdk/issues/30288

As required by
<https://material.io/guidelines/platforms/platform-adaptation.html#platform-adaptation-platform-recommendations>.

I can't figure out if this is genius or a giant hack.

This lets you use DefaultTextStyle.merge and IconTheme.merge without
specifying a BuildContext. It automatically merges in at the
appropriate place in the tree using a Builder widget.

Aggressively apply the const lint.

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

* Change how navigator prevents redundant operations

Instead of requiring transactions, we now cancel all active pointers that are
interacting with the navigator and absorb future pointers until we get a chance
to build. This approach isn't perfect (e.g., events that trigger off the
cancelled pointers could still interact with the navigator), but it should be
better than the current transaction-based approach.

Fixes #4716

* Remove openTransaction

* test

* fixup