Document that concurrent modification is bad, m'kay.

sdk/lib/core/set.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
  * to be in the set, or to _not_ be in the set.
  *
  * Set implementations may consider some elements indistinguishable. These
  * elements are treated as being the same for any operation on the set.
  *
  * The default `Set` implementation, [HashSet], considers objects
  * indistinguishable if they are equal with regard to [Object.operator==].
  *
  * Sets may be either ordered or unordered. [HashSet] is unordered and doesn't
  * guarantee anything about the order that elements are accessed in by
  * iteration. [LinkedHashSet] iterates in the insertion order of its elements.
+ *
+ * It is generally not allowed to modify the set (add or remove elements) while
+ * an operation on the set is being performed, for example during a call to
+ * [forEach] or [containsAll]. Nor is it allowed to modify the set while
+ * iterating either the set itself or any `Iterable` that is backed by the set,
+ * such as the ones returned by methods like [where] and [map].
  */
 abstract class Set<E> extends IterableBase<E> implements EfficientLength {
   /**
    * Creates an empty [Set].
    *
    * The created `Set` is a [LinkedHashSet]. As such, it considers elements that
    * are equal (using `==`) to be indistinguishable, and requires them to
    * have a compatible [Object.hashCode] implementation.
    */
   factory Set() = LinkedHashSet<E>;
@@ skipping 97 matching lines @@
    * That is, the returned set contains all the elements of this `Set` that
    * are not elements of [other].
    */
   Set<E> difference(Set<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 }