Turns out most things calling `Substitution.fromMap` first creates a map
of size 1, in which case a map doesn't seem like the right thing to use.
Also replace a few `growable: true` with `growable: false`.
When compiling the CFE with the CFE with an AOT snapshot, this CL:
instructions:u: -0.4582% +/- 0.0019% (-97936268.60 +/- 405215.88)
Change-Id: Ia1f5221cf84cd763139fcc5bd8518cbe7e575dfd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375503
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Jens Johansen
9cfb44e053