add support for dev_transformers

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#14187

---

Brief summary of a discussion we had here in person:

polymer transformers need to read the transitive closure of html files imported from an entry point. This may include assets from other packages. If those packages do not include the polymer transformer, then it's great - we'll be able to read the original assets contents.

It's likely that developers of polymer libraries might want to test their code using polymer's transformers, so we might need a notion of dev_transformers so that those transformations are not visible to other packages.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

---

Marked this as blocking dart-archive/polymer-dart#368.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

---

quick update/summary of some discussions in person

We recently started running into this problem today (see issue dart-archive/polymer-dart#368).

We used to generate code that was still OK if you applied the polymer transfomer on a library, but a recent change broke that.

We chatted a bit more about this today and discussed some other alternatives, like treating web/ and example/ as subpackages. We concluded that we would like to pursue dev_transformers first. One challenge we discovered is how to combine 'transformers' and 'dev_transformers', but we believe that concatenating the lists might be good enough.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

---

we also mention that we wanted to expose a mechanism to declare whether a Transformer is meant to be used in libraries or apps. The idea is to detect if a transformer/transformer group is used correctly.

For example, if polymer is marked as 'app-only', then if you have:

  transformers: [polymer]

you get a warning, but if you use:

  dev_transformers: [polymer]

you don't.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

---

Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

---

Merging this with the other related bug.

---

Added Duplicate label.
Marked as being merged into dart-archive/barback#39.