The new flutter.mojo is ready for use.

This means we no longer depend on the dart 1.13 beta SDK.

Add a test for signing/verifying with a random generated key pair.

Also cleaned up the flx code a bit. Replaced custom KeyPair class with
cipher's AsymmetricKeyPair.

This changes flutter's updater package to depend on and use the new flx
package. I also did a little cleanup of the Bundle interface.

This will be shared by sky/updater and flutter_tools. Currently
bundle.dart and signing.dart are unmodified copies of the files from
updater and flutter_tools, respectively. This is so future changes are
easier to see.