Support creating output directories that only contain symlinks

[jakemac53]

Closes #1242

Added a new --symlink option, which is defaulted to true for test but no other commands.

This is unfortunately a breaking change in build_runner and build_runner_core as it required adding a new interface which must be implemented by any RunnerAssetReader, which is an exposed class from both of these packages.

[jakemac53]

In terms of performance, when testing with the pub package and the new vm compilation pipeline it takes creating the merged directory from >40 seconds to <1 second. Note that the output directory is unnecessarily large since we don't clean up the modular dill files in that case, or the original dart files, but its still a good proxy I think for how this can affect large packages.

[jakemac53]

Tried running this with angulars _tests and looks like web tests don't work 🤦‍♂️ . Investigating now... but will require a fix in package:test it looks like.

[natebosch]

maybe call out that most users don't have any reason to implement this interface and are not likely impacted.

[jakemac53]

done

[natebosch]

As discussed offline - we should hold off on committing this until we've gotten another publish of build_runner that supports Dart 2.

[jakemac53]

cc @natebosch ok I updated this with the changes we talked about.

There is now a finalizeBuild method on BuildEnvironment, and we use that to create the merged output dir. Only the IOEnvironment knows about the new asset reader interface.

[jakemac53]

Looks like symlinks don't work as expected on windows even though there is supposedly support for linking individual files according to windows docs, the VM always ends up creating a directory symlink.

[jakemac53]

Filed dart-lang/sdk#33966

[natebosch]

need to include this at

build/build_runner/lib/src/logging/std_io_logging.dart

Line 69 in a10c67f

var knownNames = const [

[jakemac53]

done