Add "contains" method to Collection.

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..038f7f1197a3be5a40dfd81c12ff36422f0e6ebb 100644
--- a/lib/html/templates/html/impl/impl_Element.darttemplate
+++ b/lib/html/templates/html/impl/impl_Element.darttemplate
@@ -21,6 +21,13 @@ class _ChildrenElementList implements List {
     return output;
   }
 
+  bool contains(Element element) {
+    for (_ElementImpl child in _childElements) {
+      if (child == element) return true;
+    }
+    return false;
+  }
+
   void forEach(void f(Element element)) {
     for (_ElementImpl element in _childElements) {
       f(element);
@@ -28,7 +35,7 @@ class _ChildrenElementList implements List {
   }
 
   List<Element> filter(bool f(Element element)) {
-    final output = [];
+    final output = <Element>[];
     forEach((Element element) {
       if (f(element)) {
         output.add(element);
@@ -159,6 +166,13 @@ 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);
@@ -174,7 +188,7 @@ class _FrozenElementList implements List {
   }
 
   List<Element> filter(bool f(Element element)) {
-    final out = [];
+    final out = <Element>[];
     for (Element el in this) {
       if (f(el)) out.add(el);
     }
@@ -465,6 +479,8 @@ class _CssClassSet implements CSSClassSet {
   // interface Iterable - END
 
   // interface Collection - BEGIN
+  bool contains(String element) => _read().contains(element);
+
   void forEach(void f(String element)) {
     _read().forEach(f);
   }