[kernel] Deduplicate FunctionType(void Function()) when loading dill
[kernel] Deduplicate FunctionType(void Function()) when loading dill

On a big internal app this saves 6.5 MB of FunctionTypes.

Change-Id: I5064f92996f615ff586a2776de6775aabc2865bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266941
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@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