"pub serve" should provide more accurate output with lazy transformers

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#16743

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Right now, when "pub serve" is run, it will print "Build completed successfully" as long as no transformers provide an error. This is misleading when lazy transformers exist, because a lazy transformer will not have run yet and may in fact produce errors. As such, it should print something like "Build completed successfully with X outstanding transforms that will be run on-demand" instead.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as blocking dart-lang/sdk#15830.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Removed Priority-Unassigned label.
Added Priority-Medium label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Users haven't seemed confused by this in practice. As more transformers become lazy, I think this is likely to provide more clutter than it's worth.

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Added NotPlanned label.