Pass eventInitDict to event constructing steps

Fixes #999.

dom.bs

@@ -839,8 +839,8 @@ method steps are:
 
 <a lt="Other applicable specifications">Specifications</a> may define
 <dfn export id=concept-event-constructor-ext>event constructing steps</dfn> for all or some
-<a for=/>events</a>. The algorithm is passed an <var>event</var> as indicated in the
-<a>inner event creation steps</a>.
+<a for=/>events</a>. The algorithm is passed an <a>event</a> <var>event</var> and an {{EventInit}}
+<var>eventInitDict</var> as indicated in the <a>inner event creation steps</a>.
 
 <p class=note>This construct can be used by {{Event}} subclasses that have a more complex structure
 than a simple 1:1 mapping between their initializing dictionary members and IDL attributes.
@@ -915,7 +915,7 @@ correct defaults.</p>
  <var>event</var> has an attribute whose <a spec=webidl>identifier</a> is <var>member</var>, then
  initialize that attribute to <var>value</var>.
 
- <li><p>Run the <a>event constructing steps</a> with <var>event</var>.
+ <li><p>Run the <a>event constructing steps</a> with <var>event</var> and <var>dictionary</var>.
 
  <li><p>Return <var>event</var>.
 </ol>