This is part 4 of a broken down version of the #140101 refactor.

This PR renames isAvailableForEnvironment to isForEnvironment and replaces a regular expression with a simple function. (The latter will change the behaviour for people with branch names like `mainly_refactors` or `chess_master_experiment` or whatever, but I'm pretty sure the old behaviour was not intended.)

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This is a reland of https://github.com/flutter/flutter/pull/143176 which was speculatively reverted in https://github.com/flutter/flutter/pull/144855 but turned out not to be the cause of the tree redness.