Use iterator, moveNext(), current.

utils/pub/utils.dart

Index: utils/pub/utils.dart
diff --git a/utils/pub/utils.dart b/utils/pub/utils.dart
index d26504d9846e3f4e176d9bc849030c742086c988..1b4d93fce182df2dc4c485fac23656136d0f83cf 100644
--- a/utils/pub/utils.dart
+++ b/utils/pub/utils.dart
@@ -74,10 +74,11 @@ List flatten(List nested) {
  * Asserts that [iter] contains only one element, and returns it.
  */
 only(Iterable iter) {
-  var iterator = iter.iterator();
-  assert(iterator.hasNext);
-  var obj = iterator.next();
-  assert(!iterator.hasNext);
+  var iterator = iter.iterator;
+  var currentIsValid = iterator.moveNext();
+  assert(currentIsValid);
+  var obj = iterator.current;
+  assert(!iterator.moveNext());
   return obj;
 }