Pull in latest pub – with change to hidden directory (.pub -> .dart_tool/pub)

[kevmoo]

dart-lang/pub@df4f2e6

If we're really going to run with this

1. Create a doc (or meta bug) to track the places that should be updated
   • stagehand
   • open source templates
   • github ignores for languages
   • site-www
2. email dart-core-eng an FYI
3. email announce@ an FYI

CC @kwalrath

[mit-mit]

Please socialize with the flutter team, as it will also impact all flutter templates, packages, and plugins.

[kevmoo]

Would like to get this in for -dev.32 – so @alexander-doroshko can validate IntelliJ support

[kevmoo]

Please socialize with the flutter team, as it will also impact all flutter templates, packages, and plugins.

@mit-mit – emailed internal teams and posted to misc - https://groups.google.com/a/dartlang.org/forum/#!topic/misc/JICjwA7AOro