add ga.js (#3022)

pkgs/dartpad_ui/web/ga.js

@@ -0,0 +1,11 @@
+(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+    (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+    m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+})(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+
+ga('create', 'UA-26406144-18', {
+    cookieFlags: 'secure;samesite=none',
+    cookieDomain: 'auto'
+});
+ga('set', 'referrer', document.referrer.split('?')[0]);
+ga('send', 'pageview');

pkgs/dartpad_ui/web/index.html

@@ -26,6 +26,7 @@
   <link href="styles/cm-dartpad-dark.css" rel="stylesheet" media="screen">
   <link href="styles/cm-dartpad-light.css" rel="stylesheet" media="screen">
   <script src="codemirror/codemirror.js"></script>
+  <script src="scripts/ga.js" defer></script>
 
   <style>
     .CodeMirror {