Put all warnings behind `--show-warnings`

[chalin]

Dartdoc has a --show-warnings flag:

Generate HTML documentation for Dart libraries.

Usage: dartdoc [OPTIONS]

-h, --help                         Show command help.
    --version                      Display the version for dartdoc.
    ...
    --show-warnings                Display warnings.
   ...

Unfortunately, not all warnings are controlled by this flag, and generating the dartdocs for Angular and the Angular Components currently (always) generates lots and lots of warnings. It would be useful if these warnings were only printed when --show-warnings was provided on the command line.

cc @kwalrath

[chalin]

I just noticed the following option:

--[no-]verbose-warnings        Display extra debugging information and help with warnings.
                                   (defaults to on)

but using --no-verbose-warnings doesn't seem to help limit the number of warning messages printed for Angular docs.

[jcollins-g]

#1891 will obsolete this issue by introducing fine-grained control of warning display for dartdoc.