The gclient checkout of the engine source tree now fails if there is no prebuilt SDK available for the Dart version being built.
Developers working on Dart changes need to be able to check out and build engine with tip-of-tree Dart and with unlanded changes to Dart. The CI builders building Flutter with tip-of-tree versions of its dependencies also need this.

This can be fixed by adding a variable 'download_dart_sdk' to engine DEPS with default value true, that can be overridden
in these cases.

The failure started with a commit that makes it a failure when downloading the engine source tree and the prebuilt Dart SDK is not found: #30093.

I am uploading a PR with this proposed fix.