Implementing polyfills for insertAdjacent* to get them working on FF.

lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate

Index: lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate
diff --git a/lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate b/lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate
index 643e1b10d1e99580ee639f31eb455fed5cafa243..264184ae883a19f115386e7a97691a86de3c0c4a 100644
--- a/lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate
+++ b/lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate
@@ -197,6 +197,14 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     this._insertAdjacentNode(where, new DocumentFragment.html(text));
   }
 
+  void addText(String text) {
+    this.insertAdjacentText('beforeend', text);
+  }
+
+  void addHTML(String text) {
+    this.insertAdjacentHTML('beforeend', text);
+  }
+
   Future<ElementRect> get rect() {
     return _createMeasurementFuture(() => const EmptyElementRect(),
                                     new Completer<ElementRect>());