[dart2js] Canonicalize empty collections in FunctionType
[dart2js] Canonicalize empty collections in FunctionType

This reduces the number of _CompactLinkedHashSet objects by ~15% and
the number of _GrowableList objects by ~10% in Phase1.

Change-Id: I88ebe1b1c842977214985e23a1f3672ad43bdfdc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155826
Reviewed-by: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk