Deal with static methods in mixins

[arv]

When a mixin has a static method that method gets copied into the mixin target dynamically. Therefore we need to declare the type of that method. For example, for:

class Mixin extends React.Component {
    /** @param {number} x */
    static method(x) {}
}
ReactSupport.declareMixin(Mixin);
class Comp extends React.Component {}
ReactSupport.mixin(Comp, Mixin);

we generate:

/** @type {function(number)} */
Comp.method;