Add a generic run_dart_tool command

[stuartmorgan-g]

This is a generic version of the existing update_flutter_engine wrappers,
so that as more dart tools are added they don't each need a wrapper.

update_flutter_engine remains for now, but may be removed in the future
in favor of calling 'run_dart_tool update_flutter_engine' instead. The shell
script version has been changed to match the existing batch script
behavior of automatically providing --flutter_root (since now there's an
easy way for a caller who wants to provide something else to do so).
This may simplify future changes to provide a project-level engine
override (#160)

[krisgiesing]

LGTM. The %~dp0 syntax threw me off a bit; I had to look that up. I wonder if it's worth adding a comment about what that does, or if this is just something batch file readers are expected to know? But overall looks good.

[stuartmorgan-g]

It's used in all our existing batch files; I was assuming it was relatively standard. But since I'm new to batch scripting, I don't really have any context. (It seems much less crazy to me that using for to set a variable!)

Once things settle a bit with the new policy to minimize batch/shell in favor of Dart, I'll see how often they need to be touched. Hopefully they will need little to no ongoing maintenance since they will be so small, but if not I can add a bunch of comments to make it easier to maintain them regardless of familiarity level.

[krisgiesing]

Yeah, it does seem standard for batch files. Given the nature of the project, we may have maintainers who are not so familiar with batch (including future-us), so it may be worth adding comments for that reason, even if it's a well known idiom among people who do this a lot.