Bump to latest Dart SDK dev build (`1.21.0-dev.0.0`).

* updates to analyzer w/ support for bazel workspaces
* removes deprecated analysis `cacheSize` variable access

The circle's offset represents the position of a navigation button within the
row of buttons.  Previously, this offset had been applied to the entire width
of the bottom bar, not just the button region.

Fixes https://github.com/flutter/flutter/issues/6046

The platform plugin now supports clipboard operations.

This patch moves RawKeyboard from mojom over to platform messages. In
the process, I've also cleaned up the interface substantially.

Currently raw key events are supported only on Android, but the
interfaces defined in this patch should scale up to multiple platforms.

I ran the test 10 times on Linux and Mac and it succeeded
consistently. Let's reenable until there are new reports that it's
flaky.

Fixes #5901

Add an option to provide a custom description to predicate
finders. Without a custom description we default to printing the
predicate function's signature, which is not all that useful.

Use this new option in the driver extension to print the text of the
sought after tooltip.

Fixes #4261

Fixes #616

Also, update the docs to point to the correct property.

Fixes #6224

Developers need to get the size of the BuildContext sufficiently often
that we should provide a convenient getter for the value. Having this
getter is also an opportunity to catch common mistakes and provide
useful error messages that guide developers towards better patterns.

Fixes #2321

Fixes #6245

Fixes #6209

Fixes #773

Previously, we were nulling out the ArenaEntry in MultiDragPointerState
too early, which was prevent MultiDragPointerState from rejecting the
gesture in `dispose` if we hadn't accepted by the time the pointer went
up. Now we properly reject the gesture, which causes the tap gesture to
win during `sweep` in the arena.

Also, add a bunch of docs and annotations based on information I learned
while studying this issue. Finally, rename a private member of tap
recognizer to a name that would have confused me less in my
investigation.

Fixes #1186

When I changed how routes complete their futures, I broke the Dropdown
button because it was still waiting for its own Completer to complete
instead of using the Future returned by push. This patch fixes that
issue.

I've also removed the previous behavior of the DropdownButton forwarding
its text style to its route. The mechansim that we were using doesn't
work properly in all cases. For example, if the DropdownButton is a
child of a LayoutBuilder, then the route will have already built by the
time the DropdownButton gets a chance to forward its text style, causing
an assert in setState.

Finally, I've tweaked PopupMenuButton to work the same way as
DropdownButton in a couple corner cases (e.g., not calling the changed
callback if the button was removed from the tree before the menu
completed its Future).

Fixes #6352

Fixes #6351

Previously we would maroon the feedback in the overlay. Now we let the
drag proceed and only tear down the gesture recognizer when all the
drags it spawns have been resolved.

Fixes #6151

This patch also makes it possible for a single DragTarget to handle
multiple draggables with the same data without asserting.

Fixes #6086

Fixes #6084

Previously we were relying on the gesture arena to call us back to
cancel our timer. However, in the case where we've already been
accepted, asking the gesture arena to reject us doesn't lead to a
callback and we fail to stop the timer (and hence trigger an assert).

Fixes #6156

These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.

Fixes #5283

When not dismissing, the Dismissable widget should cull its background.
When a dismiss is in progress, it should clip the background to just the
part that is revealed.

Fixes #6127

Ensure that any new/changed doctor checks are evaluated and reported
post upgrade.

Fixes #6311

Previously we assumed the dial was square, which isn't always the case.

Fixes #6315

I added some documentation to Input as I was learning how it worked
(and some minor tweaks, like handling reparenting better).

* Add a maxLines parameter for multiline Input.

If maxLines is 1, it's a single line Input that scrolls horizontally.
Otherwise, overflowed text wraps and scrolls vertically, taking up at
most `maxLines`.

Also fixed scrolling behavior so that the Input scrolls ensuring the
cursor is always visible.

Fixes https://github.com/flutter/flutter/issues/6271

* oops

* comments

* import

* test and RO.update fix

* constant

* fix.caretRect

Previously it implied that you had to always say lib/main.dart in the "flutter run" command.