view items should support attributes and conditional compilation

[brson]

I've jumped through hoops a few times trying to write use statements conditionalized by platform.

[brson]

I've changed my mind about this.

[marijnh]

Why? With this issue in mind, I just implemented attributes for view items (I was messing with the view item parsing code anyway to add support for pub imports). But supporting conditional compilation for view items seems like a no-brainer -- it's useful, and unproblematic.

[brson]

I only changed my mind because I had a reasonable workaround and didn't want to bother with an RFC.

[graydon]

Yeah, it seems reasonable to me too.

[jdm]

Is this complete, or is conditional compilation still required?

[brson]

The configuration (conditional compilation) pass still ignores view item attributes.

[brson]

Fixed.