Export the is2D flag.

Author: tabatkins

geometry/Overview.bs

@@ -847,7 +847,7 @@ The following attributes provide status information about <a interface>DOMMatrix
 <dl dfn-type=attribute dfn-for=DOMMatrixReadOnly>
   <dt><dfn>is2D</dfn></dt>
   <dd>
-    <p>Every <a interface>DOMMatrixReadOnly</a> object must be flagged with a boolean <dfn dfn-type=dfn>is2D</dfn>. This flag indicates that
+    <p>Every <a interface>DOMMatrixReadOnly</a> object must be flagged with a boolean <dfn dfn-type=dfn export>is2D</dfn>. This flag indicates that
       <ol>
         <li>The current matrix was initialized as a 2D matrix. See individual <a href="#dommatrix-create">creators</a> for more details.</li>
         <li>Only 2D transformation operations were applied. Each <a href='#mutable-transformation-methods'>mutable</a> or <a href='#immutable-transformation-methods'>immutable transformation method</a> defines if <a>is2D</a> must be set to <code>false</code>.</li>