This reverts commit f00e06c95b.
Although Node Buffers are not covered by the tests, it seems that there
are users relying on them, so we need to add back in support for them.
This fixes issue #4359.
The Closure compiler complains about Buffer since that class exists only
in Node. That logic does not seem to be needed (unit tests and conformance
tests pass without it), so let's just remove it to solve the problem.
