Bug 835417 part 1.  Mark Node.namespaceURI as not throwing, since [Constant] things aren't allowed to throw (though we were not enforcing that).  r=peterv
Bug 835417 part 2.  Implement WebIDL parser and codegen support for marking things pure.  r=peterv
Bug 835417 part 3.  Add the notion of aliasing DOM stuff to MIR and flag MGetDOMProperty with the right alias set if it's pure.  r=jandem
Bug 835417 part 4.  Flag a bunch of DOM getters as [Pure].  r=peterv