Moved most of html lib generating scripts into tools.

tools/dom/templates/html/dart2js/impl_MouseEvent.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 /// @domName $DOMNAME
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME(String type, Window view, int detail, int screenX,
       int screenY, int clientX, int clientY, int button, [bool canBubble = true,
@@ skipping 28 matching lines @@
       // Firefox does not support offsetY.
       var target = this.target;
       if (!(target is Element)) {
         throw new UnsupportedError(
             'offsetY is only supported on elements');
       }
       return this.clientY - this.target.getBoundingClientRect().top;
     }
   }
 }