Develop1709 (#2358)

* Remove Python references in Dart math generator
Resolves #2329
Commit in develop branch

* first commit for Thomas

* Created entry for custom-dialogs
Part of issue #1709

* icon.png for custom dialog demo added

demos/index.html

@@ -137,6 +137,18 @@
       </td>
     </tr>
 
+    <tr>
+      <td>
+        <a href="custom-dialogs/index.html">
+          <img src="custom-dialogs/icon.png" height=80 width=100>
+        </a>
+      </td>
+      <td>
+        <div><a href="custom-dialogs/index.html">Custom Dialogs</a></div>
+        <div>Override Blockly browser dialogs with custom implementations.</div>
+      </td>
+    </tr>
+
     <tr>
       <td>
         <a href="storage/index.html">