Hack around absence of String.codePointAt on PhantomJS

I've been trying to get some tests working under PhantomJS, which appears not
to support String.codePointAt (which is, to be fair, an ES6 addition). For our
limited usecase, it's easier to implement the functionality from first
principles than to try to polyfill support.

src/components/views/avatars/BaseAvatar.js

@@ -99,15 +99,36 @@ module.exports = React.createClass({
         }
     },
 
-    _getInitialLetter: function() {
-        var name = this.props.name;
-        //For large characters (exceeding 2 bytes), this function will get the correct character.
-        //However, this does NOT get the second character correctly if a large character is before it.
-        var initial = String.fromCodePoint(name.codePointAt(0));
+    /**
+     * returns the first (non-sigil) character of 'name',
+     * converted to uppercase
+     */
+    _getInitialLetter: function(name) {
+        if (name.length < 1) {
+            return undefined;
+        }
+
+        var idx = 0;
+        var initial = name[0];
         if ((initial === '@' || initial === '#') && name[1]) {
-            initial = String.fromCodePoint(name.codePointAt(1));
+            idx++;
+        }
+
+        // string.codePointAt(0) would do this, but that isn't supported by
+        // some browsers (notably PhantomJS).
+        var chars = 1;
+        var first = name.charCodeAt(idx);
+
+        // check if it’s the start of a surrogate pair
+        if (first >= 0xD800 && first <= 0xDBFF && name[idx+1]) {
+            var second = name.charCodeAt(idx+1);
+            if (second >= 0xDC00 && second <= 0xDFFF) {
+                chars++;
+            }
         }
-        return initial.toUpperCase();
+
+        var firstChar = name.substring(idx, idx+chars);
+        return firstChar.toUpperCase();
     },
 
     render: function() {
@@ -116,7 +137,7 @@ module.exports = React.createClass({
         var imageUrl = this.state.imageUrls[this.state.urlsIndex];
 
         if (imageUrl === this.state.defaultImageUrl) {
-            var initialLetter = this._getInitialLetter();
+            var initialLetter = this._getInitialLetter(this.props.name);
             return (
                 <span className="mx_BaseAvatar" {...this.props}>
                     <span className="mx_BaseAvatar_initial" aria-hidden="true"