Sdk

This CL integrates the Dart SDK into spark. Specifically:
 * it adds a build step to copy the dart-sdk/lib directory into spark/app/sdk/lib
 * it adds a class to introspect the sdk from the app, and retrieve sdk contents
 * adds tests for the above

There is a caveat before this can safely land, that developers have to set the app/sdk directory to 'Don't analyze'.

@keertip @dinhviethoa