Remove unnecessary type annotation.

It seems that after switching to null safe version of package:html
we don't need it anymore.

Change-Id: I599ffb2e1b716358e07e4884544b62a2dfa72216
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181461
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer_utilities/lib/html.dart

@@ -24,7 +24,7 @@ bool containsOnlyWhitespace(Iterable<dom.Node> nodes) {
 String innerText(dom.Element parent) {
   var buffer = StringBuffer();
   void recurse(dom.Element parent) {
-    for (dom.Node child in parent.nodes) {
+    for (var child in parent.nodes) {
       if (child is dom.Text) {
         buffer.write(child.text);
       } else if (child is dom.Element) {