Fixes https://github.com/flutter/devtools/issues/9786
See go/moving-devtools-to-dart-sdk-2025.
In this change, we make the source of devtools be configurable, in
actions like build_sdk.
If `build_devtools_from_sources` is true, we build local devtools,
and if false, we continue to use the prebuilt sources.
This may be an intermediate step, while we test out building devtools
from source. Or it may be permanently be a choice, if we keep building
with CIPD.
Change-Id: I7b46d6359c69b34f316e59dccc475e211a18f965
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/498640
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Sam Rawlins
d0d8184d1a