Consider parenthesized expressions with JSDoc to be a form of type assertions

[DanielRosenwasser]

To quote @weswigham on #4447 (comment):

in [the] closure compiler you do inline casts with jsdoc comments like so:

var x = /** @type {Foo.Bar} */ (y);

We should consider supporting this in Salsa.

[weswigham]

Also type assertions in the form:

/** @type {Foo.Bar} */ var x = y;

and (in constructors:)

/** @type {Foo.Bar} */
this._foo = undefined;

which I believe are also both recognized as type annotations by the closure compiler.