Allow modal dialogs

things like prompt() and alert() are useful for debugging, learning, and education

package.json

@@ -95,7 +95,7 @@
         "c9.ide.newresource": "#981a408a7b",
         "c9.ide.openfiles": "#7fa4a97fed",
         "c9.ide.preview": "#c530a7978d",
-        "c9.ide.preview.browser": "#c50007ebbc",
+        "c9.ide.preview.browser": "#cec211a76a",
         "c9.ide.preview.markdown": "#ab8d30ad9f",
         "c9.ide.pubsub": "#a85fb27eca",
         "c9.ide.readonly": "#c699cb172b",