Provide mechanism for transformers to validate parameters

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2152569?v=3" align="left" width="96" height="96"hspace="10"> Issue by blois
Originally opened as dart-lang/sdk#20170

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Angular's transformers need a complex chunk of code to read and validate transformer parameters from the pubspec.yaml file, and generates not-too-helpful errors-
https://github.com/angular/angular.dart/blob/master/lib/transformer.dart#L64

It would be great if this were provided by BarbackSettings and could give users much better error messages (with location in the pubspec file?)

Polymer has a very similar validation as well (and all of them are currently just printing errors).

[DartBot]

<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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As we discussed in person, throwing errors rather than just printing them to stdout will cause them to be handled gracefully by pub and associated with the configuration being processed.

There's probably room here for a json_rpc_2-style API for easily validating the presence and type of values, though. See http://www.dartdocs.org/documentation/json_rpc_2/0.1.0/index.html#json_rpc_2/json_rpc_2.Parameters for example.