ExpressionDecomposer: Don't back off as much for externs properties

See: https://github.com/google/closure-compiler/blob/0a776982280072be509c2253ed33f8aef82843f7/src/com/google/javascript/jscomp/ExpressionDecomposer.java#L813-837

It looks like we're backing off an optimization because of a bug/quirk in "Internet Explorer" (it doesn't say which versions). If we can identify exactly which versions of IE have this problem then we can either remove this special case (if it's only versions we don't care about) or give users an option to optimize this more aggressively if they know it's only versions they don't care about).