Collapse together errors of the same kind

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#20424

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It's common to see one warning/error repeated in many places. For example the href vs _href might happen a lot now that we introduce the warning for it. It would be great to combine all occurrences of the same error together.

Some ideas:

• for the log_injector UI, we can combine it graphically.
• for the pub-build/pub-serve output in stdout, we can delay reporting warnings until the end of the phase, collect all warnings together and report them as such. At least we can do that on a file-per-file basis.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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It would also be great to sort errors by severity - all warnings are right now treated equal, but we probably want to rank them in some order.

This order might not be entirely static. For example, consider these 3 errors:
 a- there is an error of a broken import
 b- an warning in a template
 c- an warning that an element definition was not found.

I'd rank them in that order, because it's common that (c) is a consequence of (a). But if 'a' error is not there, I'd sort (c) before (b).

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Comment #­1 seems big enough to deserve a separate bug. I moved it to issue dart-lang/sdk#6.