Miscellaneous warnings enhancements

[jcollins-g]

While working on #1418 I needed better warning information to trace down what changed when we upgraded analyzer. This is the result. (Examples are from angular2. @chalin)

Unresolved doc references now properly indicate when the documentation is actually inherited from something else (#1405).

 warning: unresolved doc reference [AssetId]
    from angular2.transform.reflection_remover.dart.ReflectionRemover.declareOutputs: (file:///usr/local/google/home/jcollins/dart/angular2/lib/transform/reflection_remover.dart:60:3)
    in documentation inherited from barback.transformer.declaring_transformer.DeclaringTransformer.declareOutputs: (file:///usr/local/google/home/jcollins/.pub-cache/hosted/pub.dartlang.org/barback-0.15.2+10/lib/src/transformer/declaring_transformer.dart:27:3)

For some cases, the no canonical library found warning will now indicate which element referenced it. (More cases need to be added here, but this is still an improvement.)

 warning: no canonical library found for dart-html.Element.innerHtml, not linking
    from dart-html.Element.innerHtml: (file:///usr/local/google/home/jcollins/dart/all_sdks/1.24.0-dev.4.0/lib/html/dartium/html_dartium.dart)
    referred to by angular2.security.SafeInnerHtmlDirective: (file:///usr/local/google/home/jcollins/dart/angular2/lib/src/security/safe_inner_html.dart:34:7)

[devoncarew]

I think you want just a line comment here (//).

[devoncarew]

This is existing code, but this comment should probably be a doc comment (///).

[devoncarew]

lgtm!

In a future PR, we may want to translate the path urls (file:///usr/local/google/home/jcollins/dart/angular2/lib/src/security/safe_inner_html.dart) do something slightly prettier (lib/foo/bar.dart, package:foo/bar.dart, dart:core, ...). Perhaps the analysis context could help us to get back the user facing path?

[jcollins-g]

Looks like some sort of VM bug. Collecting data, but sadly the bug seems to appear rather asynchronously with anything particular happening in the code.

[jcollins-g]

vm bug filed at dart-lang/sdk#29620, holding off on this PR until that's settled.

[devoncarew]

I had the bot re-run the build - looks like the crash is consistent (which is surprising to me - I'd assumed it was an intermittent failure).

[jcollins-g]

Yep. Crash reliably happens. I have a workaround, detailed in this comment, but in order to keep the reproduction case around, leaving this branch as is until they have a chance to fix the problem.

[jcollins-g]

Going to close this PR and preserve the branch until the vm folks have a chance to fix their bug. The workaround is actually quite reasonable and is probably what dartdoc should be doing anyway, so will reopen this change in a new branch.