Add documentation for mutating operations of unmodifiable maps.

sdk/lib/collection/maps.dart

 // Copyright (c) 2012, 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.collection;
 
 /**
  * Base class for implementing a [Map].
  *
  * This class has a basic implementation of all but five of the members of
@@ skipping 138 matching lines @@
     return false;
   }
 
   V get current => _current;
 }
 
 /**
  * Mixin that overrides mutating map operations with implementations that throw.
  */
 abstract class _UnmodifiableMapMixin<K, V> implements Map<K, V> {
+  /** This operation is not supported by an unmodifiable map. */
   void operator[]=(K key, V value) {
     throw new UnsupportedError("Cannot modify unmodifiable map");
   }
+  /** This operation is not supported by an unmodifiable map. */
   void addAll(Map<K, V> other) {
     throw new UnsupportedError("Cannot modify unmodifiable map");
   }
+  /** This operation is not supported by an unmodifiable map. */
   void clear() {
     throw new UnsupportedError("Cannot modify unmodifiable map");
   }
+  /** This operation is not supported by an unmodifiable map. */
   V remove(Object key) {
     throw new UnsupportedError("Cannot modify unmodifiable map");
   }
+  /** This operation is not supported by an unmodifiable map. */
   V putIfAbsent(K key, V ifAbsent()) {
     throw new UnsupportedError("Cannot modify unmodifiable map");
   }
 }
 
 /**
  * Wrapper around a class that implements [Map] that only exposes `Map` members.
  *
  * A simple wrapper that delegates all `Map` members to the map provided in the
  * constructor.
@@ skipping 164 matching lines @@
       map[keyIterator.current] = valueIterator.current;
       hasNextKey = keyIterator.moveNext();
       hasNextValue = valueIterator.moveNext();
     }
 
     if (hasNextKey || hasNextValue) {
       throw new ArgumentError("Iterables do not have same length.");
     }
   }
 }