Generate contains() method for list mixin only if not already defined.

lib/html/src/_Collections.dart

Index: lib/html/src/_Collections.dart
diff --git a/lib/html/src/_Collections.dart b/lib/html/src/_Collections.dart
index 785e669fd2a927b99190edfc782d6ea6bee55d7b..9096fc6b2dcc2f80a244840103dbf0bf117385ff 100644
--- a/lib/html/src/_Collections.dart
+++ b/lib/html/src/_Collections.dart
@@ -8,6 +8,13 @@
  * method.
  */
 class _Collections {
+  static bool contains(Iterable<Object> iterable, Object element) {
+    for (final e in iterable) {
+      if (e == element) return true;
+    }
+    return false;
+  }
+
   static void forEach(Iterable<Object> iterable, void f(Object o)) {
     for (final e in iterable) {
       f(e);