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

lib/html/templates/html/impl/impl_Element.darttemplate

Index: lib/html/templates/html/impl/impl_Element.darttemplate
diff --git a/lib/html/templates/html/impl/impl_Element.darttemplate b/lib/html/templates/html/impl/impl_Element.darttemplate
index 6f752320501b4b58b42695a160de7e8cefe59e74..dc39ade25aa629891d31a4bae80e12b04b5fa2e0 100644
--- a/lib/html/templates/html/impl/impl_Element.darttemplate
+++ b/lib/html/templates/html/impl/impl_Element.darttemplate
@@ -21,6 +21,8 @@ class _ChildrenElementList implements List {
     return output;
   }
 
+  bool contains(Element element) => _childElements.contains(element);
+
   void forEach(void f(Element element)) {
     for (_ElementImpl element in _childElements) {
       f(element);
@@ -38,7 +40,7 @@ class _ChildrenElementList implements List {
   }
 
   bool every(bool f(Element element)) {
-    for(Element element in this) {
+    for (Element element in this) {
       if (!f(element)) {
         return false;
       }
@@ -47,7 +49,7 @@ class _ChildrenElementList implements List {
   }
 
   bool some(bool f(Element element)) {
-    for(Element element in this) {
+    for (Element element in this) {
       if (f(element)) {
         return true;
       }
@@ -159,6 +161,13 @@ class _FrozenElementList implements List {
     return _nodeList[0];
   }
 
+  bool contains(Element element) {
+    for (Element el in this) {
+      if (el == element) return true;
+    }
+    return false;
+  }
+
   void forEach(void f(Element element)) {
     for (Element el in this) {
       f(el);