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.

These classes are largely independent of the Package class.

This helps ensure that tests behave the same in environments (such as
Travis) where PUB_ENVIRONMENT is already set.

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.

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.

BUG=
R=jakemac@google.com

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

Closes #1539

On mac `Directory.systemTemp` starts with `/var` which is a symlink to `/private/var`, but the temp dir created by the ScratchSpace has the full resolved path (`/private/var/...`).

* 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>>

Also delete temp_environment which snuck back in during a bad merge

Some additional updates based on testing with PPW (now that we have an e2e to play with):

* add sourcemaps to output and update tests
* use strong mode summaries, support filenames with dots, add a fake sourcemap to satisfy package test
* add better error if the module reader fails to find a module for a file
* stop diet parsing, remove --unsafe-force-compile
* add environment constant support to the DartDevcModuleTransformer
* don't output ddc resources in folders with no entrypoints
* don't create sourcemaps in release mode
* update relevant compiler tests to run with dartdevc

Compiles js modules with dartdevc based on `.moduleConfig` files and the corresponding linked summaries from the `LinkedSummaryTransformer`.

Creates linked summaries for modules based on .moduleConfig files, using unlinked summaries from the previous step.

Closes #1567

* add ScratchSpace class which manages creating temp environments for running external binaries
* add analyzer worker driver and bazel_worker dependency
* add the UnlinkedSummaryTransformer
* rename modules after creation so the names arent so long
* Add UnlinkedSummaryTransformer to packages, use file: uris for analyzer cli

This transformer creates .moduleConfig files under each top level directory in a package, using the `computeModules` function.

Throws UnimplementedError if passing dartdevc today

* add `_ModuleComputer` class and top level `computeModules` function.
  * uses tarjans algorithm to create strongly connected components first
  * then groups modules based on the entry points that import them
* move InMemoryModuleConfigManager to util.dart
* add equalsModule as a proper matcher, and makeAssets test utility
* add isPart function to lib/src/dart.dart and add proper support for part files

Add Module/ModuleId/ModuleReader classes for storing/reading module config information