Add documentation for mutating operations of unmodifiable maps.

sdk/lib/collection/maps.dart

Index: sdk/lib/collection/maps.dart
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index c928287bd31b9351cddcff3e2566cf716ab59a87..d8dfcfccc9eb7af1c341fc9fd423a8c193d65d49 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -156,18 +156,23 @@ class _MapBaseValueIterator<K, V> implements Iterator<V> {
  * 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");
   }