Pub should more efficiently serialize barback assets

[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#14430

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Currently when pub passes a barback asset between isolates, the contents of the asset is always streamed from one isolate to the other. There are two important optimizations that could be made here:

* The cross-isolate streaming could be kicked off only when the destination isolate tries to read the asset's contents, to prevent data being passed along that will go unused.

* FileAssets could be passed by name and loaded natively within the isolate accessing the file rather than piped from the main isolate.

[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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dart-lang/sdk@c2790ca implements the first optimization and dart-lang/sdk@88124b3 implements the second.

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Added Fixed label.