add source-map support for the observable transfomer

[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#12340

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This is currently

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

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(err hit enter too quickly)

This is currently blocked on having support for source-maps or composition in barback (see issue dart-archive/barback#6).

Once we have a good way to serve intermediate assets we can then add this to the apply function:

      var filename = path.basename(id.path);
      printer.add('\n//# sourceMappingURL=$filename.map');
      printer.build(url); // where url matches the barback url of the source asset.
      transform.addOutput(
          new Asset.fromString(id.addExtension('.map'), printer.map));

And fix tests accordingly.

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Marked this as being blocked by dart-archive/barback#6.

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

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Added this to the M8 milestone.

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

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Removed this from the M8 milestone.
Added this to the Later milestone.

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

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Removed this from the Later milestone.
Added this to the M9 milestone.

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<img src="https://avatars.githubusercontent.com/u/8616782?v=3" align="left" width="48" height="48"hspace="10"> Comment by clayberg

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Removed this from the M9 milestone.
Added this to the 1.1 milestone.

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<img src="https://avatars.githubusercontent.com/u/1081711?v=3" align="left" width="48" height="48"hspace="10"> Comment by jmesserly

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I don't think we need to do this any time soon. We're not altering line numbers so debugging should be okay. And we don't transform except when compiling to JS so it generally doesn't matter

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Removed this from the 1.1 milestone.
Added this to the Later milestone.
Added NotPlanned label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Observe label.
Added Pkg-Observe label.