Dart_eval: Interoperable Dart Code Evaluation

dart_eval's primary goal is to be interoperable with real Dart code. Classes created in 'real Dart' can be used inside the interpreter with a wrapper, and classes created in the interpreter can be used outside it by creating an interface and bridge class.

dart_eval's compiler is powered under the hood by the Dart analyzer, so it achieves 100% correct and up-to-date parsing (although compilation and evaluation aren't quite there yet.)

Currently dart_eval implements a decent amount of the Dart spec, but there are still missing features like generators, Sets and extension methods. In addition, much of the standard library hasn't been implemented.