Pub should more efficiently serialize barback assets

[nex3]

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.

[nex3]

Marked this as blocking #13848.

[nex3]

r29408 implements the first optimization and r30126 implements the second.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#682.