[Fuchsia] Remove the use of //build/ in fuchsia

[zijiehe-google-com]

Motivation of the change:

Both dart and flutter are using fairly outdated gn-sdk without properly maintained. Currently @hjfreyer is working on version'ed IDK / SDK libs which requires changes in gn-sdk to use the right version of the libs in fuchsia/sdk/obj/{arch}-api-{level} rather than the one in the fuchsia/sdk/arch. But current implementation does not support choosing the right version.

Blocking issue:

The new gn-sdk (in flutter/tools/fuchsia/gn-sdk) generates multiple BUILD.gn files rather than a large BUILD.gn the previous version created. So most of the build rules need to switch from the old fidl:{api} build rule to fidl/{api} rule. The same change will happen in the dart/sdk, i.e. http://go/dart-reviews/356924. But since the two repos cannot have one single atomic change, changing either side first will cause flutter to break. E.g. the linkage error caused by duplicated symbols will happen if we change the dart/sdk first, since in flutter, it will still refer to the old build rules in the middle.

Solutions:

Ideally we can create redirect rules in the current build/fuchsia buildroot tree to redirect the old rules into the new one, so we can make the change in the flutter first then dart/sdk. But creating the rules is not trivial and will only be used once.

So an alternative solution is

• pause the dart/sdk -> flutter roll
• submit dart/sdk change (http://go/dart-reviews/356924)
• update this change to manually bring the dart/sdk change, namely the dart_revision in the DEPS file and signatures in the ci/licences.
• resume the dart/sdk -> flutter roll.

But it requires this change itself to be reviewed first, and I'd like to know your opinion before moving forward.

See corresponding dart/sdk change at http://go/dart-reviews/356924.

Bug: b/40935282

FYI: @hjfreyer

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the CLA.
• [x] All existing and new tests are passing.

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