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

Fix #1560.

Fixes #1336

Record when we log and error and mark the entire build as failed. This
catches the cases where there are errors on assets that did not
transitively impact dart2js output - like transformers on non-Dart
files.

The current ignore is too broach and matches some of the tests. A
leading slash makes the ignore only match a build directory at the root
of the repo

Just like test and tool
Also expanded and generalized these tests

Closes https://github.com/dart-lang/pub/issues/1537

Fixes #1535

The tests execute `git` in a number of places which is impacted by the
users global git config and can cause failure.

- Remove `info` and `hooks` from the list of required directories under
  `.git`. These are controlled by the `init.templatedir` config and are
  only the defaults but may not exist.
- For git repos created during the test configure `core.excludesfile` as
  empty since we rely on having full control over ignored files when we
  create `.gitignore` ourself.

Use a home-rolled version

* Add metadata to requests to pub.dartlang.org.

This will help pub.dartlang.org better track how packages relate to one
another.

* Fix broken depenency tests.

Those unit tests import pub libraries directly without invoking a
command, so command was throwing an exception. Worse, the Pokemon
exception handler in DependencyValidator was then catching the
StateError, hiding the bug.

Also applied some other changes I suggested.

Get the Mac tests passing again with the mtree stuff in tar.

run dartfmt

Closes #1519

"git ls-files" doesn't recurse into submodules, so we need to do so
manually.

Closes #1523

Fixes #1442

I'm not sure why this wasn't causing us problems before.