Support method/function overloads

This has been discussed periodically both in the issue tracker and in person, but I don't think there's a tracking issue yet. (**EDIT**: Here is the original issue from 2011 - https://github.com/dart-lang/sdk/issues/49).

Now that we're moving to a sound type system, we have the ability to overload methods—that is, to choose which of a set of methods is called based on which arguments are passed and what their types are. This is particularly useful when the type signature of a method varies based on which arguments are or are not supplied.