Fixing CSS Animations cross-platform issues.

tools/dom/templates/html/impl/impl_WebKitCSSKeyframesRule.darttemplate

Index: tools/dom/templates/html/impl/impl_WebKitCSSKeyframesRule.darttemplate
diff --git a/tools/dom/templates/dart2js_impl.darttemplate b/tools/dom/templates/html/impl/impl_WebKitCSSKeyframesRule.darttemplate
similarity index 59%
copy from tools/dom/templates/dart2js_impl.darttemplate
copy to tools/dom/templates/html/impl/impl_WebKitCSSKeyframesRule.darttemplate
index 98c247c37d6c685f7affe94698f9625b9b2b2c90..987288cd4597af82b6078506d04994a599404bc8 100644
--- a/tools/dom/templates/dart2js_impl.darttemplate
+++ b/tools/dom/templates/html/impl/impl_WebKitCSSKeyframesRule.darttemplate
@@ -6,4 +6,15 @@ part of $LIBRARYNAME;
 
 @DocsEditable
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$MIXINS$IMPLEMENTS$NATIVESPEC {
-$!MEMBERS}
+$!MEMBERS
+
+$if DART2JS
+  void appendRule(String rule) {
+    if (JS('bool', '("appendRule" in #)', this)) {
+      JS('', '#.appendRule(#)', this, rule);
+    } else {
+      JS('', '#.insertRule(#)', this, rule);
+    }
+  }
+$endif
+}