Remove unsupported --unsafe-angular2-whitelist parameter.

BUG=
R=jakemac@google.com, vsm@google.com

Review-Url: https://codereview.chromium.org//2939453002 .

Throw real errors and handle them uniformly:

* AssetNotFoundExceptions are passed on
* All other errors are converted to AssetNotFoundExceptions after:
  * Logging the error
  * Creating a new "*.errors" asset which contains the error message
* On the frontend if require js fails to load a module it will request
  the "*.errors" file for the module and log the response.

* remove unused imports

* use SourceFile.fromString, the default constructor is deprecated

BUG=
R=rnystrom@google.com

Review-Url: https://codereview.chromium.org//2899863003 .

BUG=
R=jakemac@google.com

Review-Url: https://codereview.chromium.org//2893483005 .

* use Result instead of Future for the asset cache
* add tests for requesting invalid files that look like dartdevc files

* Migrated all shared code out of the transformer files, and remove Transform arguments
* Removed transformers, replace with a layer on top of barback
* _buildAsset can synchronously return the map of assets that will be created
* use one scratchSpace for all builds, get shared libs outside of lib working
* make ScratchSpace use the common createFileFromStream method to limit number of open files
* update return type of getBuiltInTransformers back to Iterable<Set<Transformer>>