Consider getting rid of `package:analyzer_cli`

[stereotype441]

In an internal doc where I talked about package:analyzer_cli, @bwilkerson suggested:

We should consider getting rid of this [package]. The only reason it's currently needed (that I'm aware of) is because dart analyze doesn't support the "read files to be analyzed from stdin" feature.

Note that package:analyzer_cli is a small utility library used by test.py to run automated tests (such as language tests and co19 tests) through package:analyzer. It’s not the command-line interface of the analyzer exposed to users through dart analyze.

[munificent]

A few weeks ago, I changed the static error test updater script to use analyzer as a library instead of through the dart analyze command-line, and it made the updater literally >100x faster to analyze files.

Having the test_runner use analyzer directly as a library might also significantly increase the performance of analysis tests. Probably nowhere near to the same degree because I think the test runner already uses a semi-persistent batch mode when talking to analyzer_cli, but I bet there is still overhead.