This is similar to --package-warnings, except that diagnostics will
only be reported for Dart packages matching the given prefix.
Also added the number of source files analyzed and the number of
errors generated to performance report.
Cleanup: simplified code for triggering diet parsing.
BUG=
R=paulberry@google.com
Review URL: https://codereview.chromium.org/1806263004 .
Brian Slesinsky