MDL-41030 Behat: Add support for waiting for javascript

This is so that phantomjs (which runs faster than selenium) can pass the tests.

lib/javascript-static.js

@@ -755,6 +755,76 @@ M.util.init_block_hider = function(Y, config) {
     });
 };
 
+/**
+ * @var pending_js - The keys are the list of all pending js actions.
+ * @type Object
+ */
+M.util.pending_js = [];
+M.util.complete_js = [];
+
+/**
+ * Register any long running javascript code with a unique identifier.
+ * Should be followed with a call to js_complete with a matching
+ * idenfitier when the code is complete. May also be called with no arguments
+ * to test if there is any js calls pending. This is relied on by behat so that
+ * it can wait for all pending updates before interacting with a page.
+ * @param String uniqid - optional, if provided,
+ *                        registers this identifier until js_complete is called.
+ * @return boolean - True if there is any pending js.
+ */
+M.util.js_pending = function(uniqid) {
+    if (uniqid !== false) {
+        M.util.pending_js.push(uniqid);
+    }
+
+    return M.util.pending_js.length;
+};
+
+/**
+ * Register listeners for Y.io start/end so we can wait for them in behat.
+ */
+M.util.js_watch_io = function() {
+    YUI.add('moodle-core-io', function(Y) {
+        Y.on('io:start', function(id) {
+            M.util.js_pending('io:' + id);
+        });
+        Y.on('io:end', function(id) {
+            M.util.js_complete('io:' + id);
+        });
+    });
+    YUI.applyConfig({
+        modules: {
+            'moodle-core-io': {
+                after: ['io-base']
+            },
+            'io-base': {
+                requires: ['moodle-core-io'],
+            }
+        }
+    });
+
+};
+
+// Start this asap.
+M.util.js_pending('init');
+M.util.js_watch_io();
+
+/**
+ * Unregister any long running javascript code by unique identifier.
+ * This function should form a matching pair with js_pending
+ *
+ * @param String uniqid - required, unregisters this identifier
+ * @return boolean - True if there is any pending js.
+ */
+M.util.js_complete = function(uniqid) {
+    var index = M.util.pending_js.indexOf(uniqid);
+    if (index >= 0) {
+        M.util.complete_js.push(M.util.pending_js.splice(index, 1));
+    }
+
+    return M.util.pending_js.length;
+};
+
 /**
  * Returns a string registered in advance for usage in JavaScript
  *