Defer the nacl plugin loading until it's needed.

Fixes #3120
Addressing review comments
Use 'pub upgrade' instead of 'pub get' in `grind setup`
Merge pull request #3131 from dart-lang/fix-grind_setup_boot

Use 'pub upgrade' instead of 'pub get' in `grind setup`
Merge pull request #3127 from rginda/defer-nacl-load

Defer the nacl plugin loading until it's needed.
Delay branch head update on creation of a commit.
Allow non-fast-forward git pulls.
Merge pull request #3134 from dart-lang/git_pulls

Allow non-fast-forward pull
Merge remote-tracking branch 'upstream/master' into flush-changed-preferences-immediately