Fix SVGElement#classes.

client/html/src/CssClassSet.dart

Index: client/html/src/CssClassSet.dart
diff --git a/client/html/src/CssClassSet.dart b/client/html/src/CssClassSet.dart
index 00a8aa2422277870b20f2425be68949ef9ad22be..6fc3451b7a828587cb0f0363502ef05cf9d0bb2d 100644
--- a/client/html/src/CssClassSet.dart
+++ b/client/html/src/CssClassSet.dart
@@ -117,7 +117,7 @@ class _CssClassSet implements Set<String> {
   Set<String> _read() {
     // TODO(mattsh) simplify this once split can take regex.
     Set<String> s = new Set<String>();
-    for (String name in _element.className.split(' ')) {
+    for (String name in _className().split(' ')) {
       String trimmed = name.trim();
       if (!trimmed.isEmpty()) {
         s.add(trimmed);
@@ -127,6 +127,12 @@ class _CssClassSet implements Set<String> {
   }
 
   /**
+   * Read the class names as a space-separated string. This is meant to be
+   * overridden by subclasses.
+   */
+  String _className() => _element.className;
+
+  /**
    * Join all the elements of a set into one string and write
    * back to the element.
    */