Make Iterable more powerful (and lazy).

sdk/lib/core/iterator.dart

Index: sdk/lib/core/iterator.dart
diff --git a/sdk/lib/core/iterator.dart b/sdk/lib/core/iterator.dart
index 2e324f6a9b26ead8b7d201a25dfc34f52855ebba..e6d7d0fb0321faaf2233229be83ddb1d85655ffd 100644
--- a/sdk/lib/core/iterator.dart
+++ b/sdk/lib/core/iterator.dart
@@ -9,16 +9,27 @@
  *
  * If the object iterated over is changed during the iteration, the
  * behavior is unspecified.
+ *
+ * The [Iterator] is initially positioned before the first element. Before
+ * accessing the first element the iterator must thus be advanced ([moveNext])
+ * to point to the first element. If there is no element left, then [moveNext]
+ * returns false.
  */
 abstract class Iterator<E> {
   /**
-   * Gets the next element in the iteration. Throws a
-   * [StateError] if no element is left.
+   * Moves to the next element. Returns true if [current] contains the next
+   * element. Returns false, if no element was left. It the latter case
+   * trying to read [current] will throw a [StateError].
+   *
+   * It is safe to invoke [moveNext] even when the iterator is already
+   * positioned after the last element. In this case [moveNext] has no effect.
    */
-  E next();
+  bool moveNext();
 
   /**
-   * Returns whether the [Iterator] has elements left.
+   * Returns the element the iterator is positioned at. This getter throws
+   * a [StateError] if the iterator is positioned before the first, or after the
+   * last element.
    */
-  bool hasNext;
+  E get current;
 }