Support features with "pub global activate".

Partially addresses #1593

Partially addresses #1593

This allows packages to declare features with dependencies, and to opt
in to those features in dependency declarations.

See #1593

Wait for --build-delay milliseconds between filesystem events before scheduling new builds.

This isn't going to make it into -dev.4.0.

This will make it easier to add features to the pubspec and verify that
they're only published with appropriate SDK constraints.

It also simplifies the dependency validator logic around caret
constraints, since it's a lot less likely that anyone will want to
support an SDK older than 1.8.0 at this point.

We used to name these after the package itself, but that makes it
difficult to share repositories between sub-packages.

This more clearly indicates the type of data the class contains. We
were already using in more places than just dependencies, so it makes
sense to have a name that reflects that.

These classes are largely independent of the Package class.

Remove unsupported --unsafe-angular2-whitelist parameter.

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

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

This switches pub's tests over to using the stream matchers built into
test, as well as the test_descriptor, test_process, and
shelf_test_handler packages. It uses async/await rather than
scheduled_test's task queue.

This removes a layer of indirection which should make it much easier to
work on pub's test infrastructure, especially for people unfamiliar with
scheduled_test.

...instead of info.

Allows capturing response information in the case of 5xx errors without
requiring verbose logging

Related to https://github.com/dart-lang/pub-dartlang-dart/issues/94

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 .

Closes #1539

* 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>>