Allow mixin classes to have members added via .prototype calls.

[aomarks]

For example:

  /** @type {string} */
  MyMixin.prototype.foo;

We already support this behavior for normal classes, but not for mixin classes.

We need this because we're going to change some class mixin properties such as PolymerElement._template to not be initialized in the constructor, but we still need a way to tell analyzer that they exist.

cc @sorvell @kevinpschaaf

[rictic]

We can also do this via:

/** @type {string} */
this.foo;

in the constructor, which I'd argue is more legible. I guess if that's the only thing in the constructor it might be better for the VM to leave it off though?

[aomarks]

We can also do this via:

/** @type {string} */
this.foo;

in the constructor, which I'd argue is more legible. I guess if that's the only thing in the constructor it might be better for the VM to leave it off though?

Yeah @kevinpschaaf and @sorvell specifically wanted to avoid having the property accesses at all in the constructor, for uncompiled users.