Remove lastModifiedDate handling. (#300)

Several browsers used to implement a nonstandard lastModifiedDate property on File objects. This has been removed everywhere except Chrome, and it's not in the File API spec. This specification included special cases for keypath evaluation of that property name, and it wasn't tested. Remove the references.

Closes #215

index.bs

@@ -762,7 +762,7 @@ following type-specific properties:
 <table class=props>
   <tr><th>Type</th><th>Properties</th></tr>
   <tr><td>{{Blob}}</td><td>{{Blob/size}}, {{Blob/type}}</td></tr>
-  <tr><td>{{File}}</td><td>{{File/name}}, {{File/lastModified}}, `lastModifiedDate`</td></tr>
+  <tr><td>{{File}}</td><td>{{File/name}}, {{File/lastModified}}</td></tr>
   <tr><td>[=Array=]</td><td>`length`</td></tr>
   <tr><td>[=String=]</td><td>`length`</td></tr>
 </table>
@@ -6344,10 +6344,6 @@ ECMAScript value or failure, or the steps may throw an exception.
       : If |value| is a {{File}} and |identifier| is "`lastModified`"
       :: Let |value| be a Number equal to |value|'s {{File/lastModified}}.
 
-      : If |value| is a {{File}} and |identifier| is "`lastModifiedDate`"
-      :: Let |value| be a new [=Date=] object with \[[DateValue]] internal slot
-          equal to |value|'s {{File/lastModified}}.
-
       : Otherwise
       ::
           1. If [=/Type=](|value|) is not Object, return failure.
@@ -6897,6 +6893,7 @@ For the revision history of the second edition, see [that document's Revision Hi
 * Added {{IDBFactory/databases()}} method. ([Issue #31](https://github.com/w3c/IndexedDB/issues/31))
 * Added {{IDBTransaction/commit()}} method. ([Issue #234](https://github.com/w3c/IndexedDB/issues/234))
 * Added {{IDBCursor/request}} attribute. ([Issue #255](https://github.com/w3c/IndexedDB/issues/255))
+* Removed handling for nonstandard `lastModifiedDate` property of {{File}} objects. ([Issue #215](https://github.com/w3c/IndexedDB/issues/215))
 
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 # Acknowledgements # {#acknowledgements}