The removed assertion consistently passes not because the invoked
`close` method internally throws a `TypeError` but because the `close`
method does not exist. The ES6 specification does not define a `close`
method on the GeneratorPrototype, so this test is a tautology.

BUG=None
LOG=N
R=arv

Review URL: https://codereview.chromium.org/1046963002

Cr-Commit-Position: refs/heads/master@{#27567}