Bump Dart SDK to `1.18.0-dev.2.0`.

This API is the front-end part of the work on showing licenses.

Future patches will:

* Provide an API for registering what licenses should be shown here,
  which will be used by this feature to shown licenses but could also be
  used by custom code for showing licenses (e.g. for people not using
  the Material widgets).

* Actually populate this license API from all the licenses we currently
  use in the engine, in the framework, and from any pub packages that
  are used (directly or indirectly) by the application.

Gets a bunch of fixes.  Most notably a VM that has background compilation re-enabled (and stable).  This should win us (back) a ~17% improvement of analysis times (CLI and server).

See: https://github.com/dart-lang/sdk/issues/26697 for more context.

I erroneously removed this function in an earlier patch. Also, export
SystemUiOverlayStyle because we expose that enum in the SystemChrome
API.

This required switching from the Future-based bindings to the callback-based
bindings.

This is now obsolete

The error code path wasn't actually getting the point where it would throw the
useful error message. Also, fix error recovery when runApp fails.

Fixes #4655

* Fixing sprites' image map regression.
* Smoke tests for ImageMap

* Only schedule overlay style update microtask if needed

* Simplify API

Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.

Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.

Icon itself now takes the icon as a positional argument, for brevity.

ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.

IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.

IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.

IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.

Most places that introduced an IconTheme widget now use IconTheme.merge.

IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.

ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.

Also: Fix the analyzer to actually check the stocks app.

Turns out we weren't managing focus correct between navigator routes because we
were missing a Focus widget above the routes. However, adding this widget
caused us to explode at startup because the initial route was trying to move
focus during the build phase.

This patch teaches Focus to have an initiallyFocusedScope, which can be use to
initialize the child focus scope.

Fixes #4065

In writing a test for #1927, I found a number of bugs in how Draggable shuts
down. Previously it would leak its recongizer. Now it disposes its recognizer
and the recognizer knows how to be disposed cleanly.

Fixes #1927

(onResume is now a no-op)

Fixes https://github.com/flutter/flutter/issues/4506

* System chrome overlay style fixes

1) Add a cache to avoid superfluous calls to the embedder
2) Coalesce calls to the embedder that are made in the same event loop
3) Move call site to material's AppBar from Title widget

#4461

* bump out dep on the test package

* add other pubspecs

Fixes #4553

I broke them when refactoring images. Oops.

Fixes #4169 and also now overscroll indicator is dismissed the moment
user scrolls in the opposite direction (#4603) but the bounce overscroll
that happens behind the scene and is clamped in the indicator is still
problematic and needs to be fixed. However these fixes are orthogonal to that.

Also closes #4127 as I verified the timeout feature (reduced the duration
to 500ms to be closer to Android behaviour)