Make asset consumption more explicit

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<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="96" height="96"hspace="10"> Issue by kevmoo
Originally opened as dart-lang/sdk#16612

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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

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

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<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.

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<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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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.

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<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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Issue dart-archive/barback#17 has been merged into this issue.

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

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

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<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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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.

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<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@d11e95b adds support for passing assets through a transformer by default.

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<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@2c19a00 adds the ability for transformers to explicitly consume their primary inputs.

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Added Fixed label.