Remove NoYieldInGenerator warning message.

It's fine for a generator function to contain no `yield` expressions, just
as it's fine for control flow to dynamically avoid any `yield` expressions
that do appear in the generator function. Permitting no-`yield` generator
functions is one of the stated reasons for the `function*` syntax.

https://code.google.com/p/esprima/issues/detail?id=485
Treat yield as a contextual keyword.

This means, in particular, that `yield` is a perfectly valid identifier
outside the bodies of generator functions, and that it is always treated
as an operator inside the bodies of generator functions.

When we encounter an expression like `yield x` outside a generator
function, parsing fails with an "Unexpected identifier" error pointing to
the `x`, as you would expect with `whatever x`.

https://code.google.com/p/esprima/issues/detail?id=467