Barback can run too many transformers in parallel

[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#14320

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When barback is run via "pub build" or "pub serve", it will attempt to run all applicable transformers. This can cause problematic memory consumption for large projects, especially when using heavyweight transformers like dart2js in projects with many applicable files. Barback should limit its transformer usage to avoid running out of memory.

[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 #647.

[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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Fixed in dart-lang/sdk@794dc9d.

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Added Fixed label.