covariant subtype values with @template

A couple of useful tricks using @template.

Return the calling subtype from a super class method

/** @constructor */
function Foo() {}

/** 
 * @this {THIS}
 * @return {THIS}
 * @template THIS
 */
Foo.prototype.method = function() {
  return this;
}

/** @constructor @extends {Foo} */
function Bar() {}
Bar.prototype = new Foo();

var x = new Bar().method(); // x is of type "Bar"