Previously, several commands automatically ran "pub get" implicitly before doing
their work. We've determined that this behavior is confusing, and for "pub run"
in particular, makes the executables seem much slower than they actually are.

This also allows us to make access to the entrypoint's package graph
synchronous.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1282533003 .