Functions/methods registered in `platform_message_handlers_` should return true or false, indicating whether the response was handled by them.
This makes it more explicit whether a platform message handler will asynchronously complete a response, versus if there was malformed/invalid request.

If the response was not handled, `CompleteEmpty` in `PlatformView::HandlePlatformMessage` to avoid hanging `Future`s in dart.

This fixes https://fxbug.dev/79056.

https://dart.googlesource.com/sdk.git/+log/bbd701b4ba76..fff3a3747a18

2021-06-29 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-253.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
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Import the root options and explicitly disable lints that our code
violates.

Fix several lint violations.

Context creation options for each backend were spread across multiple
translation units. This makes setting options common across all backends hard to
configure. I have moved the creation of such common options into a separate
translation unit.

Fixes https://github.com/flutter/flutter/issues/84213

This adds support for adding a cml file to a fuchsia package. The
cml file will be compiled to a cm file and included in the archive.

This reverts commit f04d2044.