Ignores @augments tag on a class without an explicit `extends`

TypeScript Version: nightly (2.6.0-dev.20170923)

Code

class A {
    a() {}
}

/** @augments {A} */
class B {
    b() {
        this.a();
    }
}

// @ts-ignore
inherit(A, B);

Expected behavior:

Either:

• An error on the @augments tag.
• The @augments tag is accepted and I can access methods from A inside B.

Actual behavior:

@augments is ignored unless we actually see extends.

Noticed while working on #18739.