Creating new Java classes from Dart

This seems to be completely out of scope from what JNIgen wants to be: a Dart bindings generator for Java, since we would have to generate Java bindings for Dart.

However we could consider adding this feature if it's useful enough. This would mean that we would have to generate both Dart code and Java bytecode from some Dart code. There are also some libraries that can dynamically generate bytecode such as [Byte Buddy](https://bytebuddy.net) or [Javassist's ProxyFactory](http://www.javassist.org/html/javassist/util/proxy/ProxyFactory.html).