Attempt pub get/upgrade before running build or serve

[nshahan]

No description provided.

[nshahan]

So the idea I was trying out here was to detect when running build_runner build or build_runner serve failed because of the need for a pub get. If webdev knows we need to run a pub get, it should just do it.

I feel this is needed for the ideal initial workflow:

• pub global activate webdev
• webdev create foo
• cd foo
• webdev serve

[nshahan]

I'm fine with abandoning this PR now but WDYT?

[kevmoo]

create is out of scope in the short term. Let's just close this PR and revisit later.