In aaa5ad2 we introduced a feature test. However though we do require
ES5, we don't (yet) use strict mode. It is an easy and solid one to
feature test, but it the unfortunate reality is that IE9 does not fully
support ES5. Strict mode is among the features it didn't (correctly)
implement. As a result commit aaa5ad2 locked out IE9 completely.
Replacing it with a more explicit (and harder to maintain) test that
does pass in IE9. We now need to be very careful about using ES5
features in our code. It has to be in this list here to avoid breaking
in a browser that doesn't support it.
Change-Id: I77b623e15a0791bfaa2cd835e43107cefb3c25f5
01f9e560b4