Make asset consumption more explicit

[kevmoo]

Scenario: I have 10 pngs files in assets.

I'd like to generate a single png sprite, with a new file name, and not include the source images in output

[munificent]

Removed Pub-Build label.
Added Pkg-Barback, Area-Pub labels.

[munificent]

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

[munificent]

From our discussion yesterday, the current plan is that we'll add a method like:

   consumeInput(AssetId id)

to Transform. Transformers can call that to explicitly prevent that asset from being forwarded.

In addition, we'll change the default behavior so that a primary input is not automatically consumed. As far as we know, changing that default won't break any existing transformers because most of those output an asset on top of the primary input, which still effectively consumes it since it gets overwritten.

[munificent]

Issue #14554 has been merged into this issue.

[nex3]

Set owner to @nex3.
Added Started label.
Changed the title to: "Make asset forwarding more explicit".

[nex3]

Changed the title to: "Make asset consumption more explicit".

[nex3]

After some discussion (see https://code.google.com/p/dart/issues/detail?id=14554#c5 for details), we've decided not to expose a general [consumeInput] function. Instead, we'll add a [consumePrimaryInput] function.

[nex3]

r33134 adds support for passing assets through a transformer by default.

[nex3]

r33557 adds the ability for transformers to explicitly consume their primary inputs.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#847.