add source-map support for the observable transfomer

[sigmundch]

This is currently

[sigmundch]

(err hit enter too quickly)

This is currently blocked on having support for source-maps or composition in barback (see issue #12339).

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 #12339.

[sigmundch]

Added this to the M8 milestone.

[sigmundch]

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

[sigmundch]

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

[clayberg]

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

[jmesserly]

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.

[anders-sandholm]

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

[DartBot]

This issue has been moved to dart-archive/observe#29.