Get rid of google charts errors.

static/externs.js

@@ -0,0 +1,14 @@
+/* Externs that we don't get from real sources are here */
+
+google.charts = {};
+
+/**
+ * @param {function()} callback
+ */
+google.charts.setOnLoadCallback = function(callback) {};
+
+/**
+ * @param {Node} container
+ * @constructor
+ */
+google.visualization.Timeline = function(container) {};

static/verify.sh

@@ -10,6 +10,7 @@ curl \
   --data warning_level=verbose \
   --data externs_url=https://www.cosmopolite.org/externs/cosmopolite.js \
   --data externs_url=https://raw.githubusercontent.com/google/closure-compiler/master/contrib/externs/google_visualization_api.js \
+  --data-urlencode "js_externs@externs.js" \
   --data-urlencode "js_code@babystats.js" \
   http://closure-compiler.appspot.com/compile | ./prettyprint.py