Document that concurrent modification is bad, m'kay.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 42ecd46652decb7d73fd399d9ac41332770d25e3..2db8b99da2ff27e5286569bb41d44d33cc7ea2b6 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -25,6 +25,11 @@ part of dart.core;
  * You can implement Iterable in your own class.
  * If you do, then an instance of your Iterable class
  * can be the right-hand side of a for-in construct.
+ *
+ * Some subclasss of `Iterable` can be modified. It is generally not allowed
+ * to modify such collections while they are being iterated. Doing so will break
+ * the iteration, which is typically signalled by throwing a
+ * [ConcurrentModificationError] when it is detected.
  */
 abstract class Iterable<E> {
   const Iterable();