Remove use of unstable box syntax.

http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.

This is expected since `Box::new` is defined as:

```rust
impl<T> Box<T> {
    #[inline(always)]
    pub fn new(x: T) -> Box<T> {
        box x
    }
}
```

With inlining, it should compile to the same as box syntax.

1 files changed, 1 insertions, 1 deletions

diff --git a/components/script/dom/htmldialogelement.rs b/components/script/dom/htmldialogelement.rs
index 39e195e0814..9762a6c3c97 100644
--- a/components/script/dom/htmldialogelement.rs
+++ b/components/script/dom/htmldialogelement.rs
@@ -37,7 +37,7 @@ impl HTMLDialogElement {
     pub fn new(local_name: LocalName,
                prefix: Option<Prefix>,
                document: &Document) -> DomRoot<HTMLDialogElement> {
-        Node::reflect_node(box HTMLDialogElement::new_inherited(local_name, prefix, document),
+        Node::reflect_node(Box::new(HTMLDialogElement::new_inherited(local_name, prefix, document)),
                            document,
                            HTMLDialogElementBinding::Wrap)
     }