[tools] Add binary size tool.

Compared to (Chromium's) third_party/binary_size, this tool better simplifies paths and can handle `dart:` and `package:` "paths".

Cf. e32d98c.

Change-Id: I9e757d9ba8422024584586899481279f7fd6542d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387840
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google