https://github.com/tc39/ecma262/pull/988 changes the iteration protocol
such that the "next" method is only loaded from the iterator object once
during the prologue of iteration, rather than during each step.

- Expand ambiguous assertion messages and assert execution paths more
  finely.
- Improve variable names in `for..of` tests
  While the object created by a GeneratorFunction may be considered an
  "iterable", it is being used as an iterator in these tests. Naming the
  variable according to the way it is used improves the readability of
  the test body.
- Add 'features' attribute to test frontmatter
- Move tests
- Introduce additional `for..of` control flow tests

These tests are derived from the following files within the Google V8
project:

test/mjsunit/es6/iteration-semantics.js