Allow default methods/instance variables in interfaces #3536

This would be epic to have. Java itself had a problem with forcing the implementing class to implement the abstract methods, and solved it with default interface implementations. Sometimes, multiple inheritance is necessary to have. For example, I have a sea of crisscrossing is-a relationships to handle, with a ton of things like the following:

class Identifier extends Node implements Assignable, ImportPatternEntry,
        IdentifierLike, PropertyLike, Subtypable {
    // stuff...
}

class MemberExpression extends Node implements Assignable,
        IdentifierLike, PropertyLike, Subtypable {
    // stuff...
}

It's not pretty, but I also have to check many of these relationships at runtime as well. I can't use traditional single inheritance for this, either, because none of these are mutually exclusive. I would love to have ways to set default properties on these classes, to where I don't have to make a build script to generate all this. (I have 30+ base classes and 20+ interfaces with very intertwined relationships like the above.)

What I was thinking of was something like this:

interface IFoo {
  bar = true;
}

class Foo {}

class Bar implements IFoo {}

new Foo().bar; // undefined
new Bar().bar; // true