Updating iterable.current comments and behavior to be consistent.

Change-Id: I57226491571272aa483aec8b5e14dc00c55511ed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132812
Commit-Queue: Mark Zhou <markzipan@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

sdk_nnbd/lib/core/iterable.dart

@@ -13,8 +13,12 @@ part of dart.core;
  * and if the call returns `true`,
  * the iterator has now moved to the next element,
  * which is then available as [Iterator.current].
- * If the call returns `false`, there are no more elements,
- * and `iterator.current` returns `null`.
+ * If the call returns `false`, there are no more elements.
+ * The [Iterator.current] value must only be used when the most
+ * recent call to [Iterator.moveNext] has returned `true`.
+ * If it is used before calling [Iterator.moveNext] the first time
+ * on an iterator, or after a call has returned false or has thrown an error,
+ * reading [Iterator.current] may throw or may return an arbitrary value.
  *
  * You can create more than one iterator from the same `Iterable`.
  * Each time `iterator` is read, it returns a new iterator,

sdk_nnbd/lib/internal/iterable.dart

@@ -572,7 +572,7 @@ class TakeWhileIterator<E> extends Iterator<E> {
   }
 
   E get current {
-    if (_isFinished) throw IterableElementError.noElement();
+    if (_isFinished) return null as E;
     return _iterator.current;
   }
 }