Allow Object when doing lookups.

runtime/lib/immutable_map.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.
 // Immutable map class for compiler generated map literals.
 
 class ImmutableMap<K, V> implements Map<K, V> {
   final _ImmutableArray kvPairs_;
 
   const ImmutableMap._create(_ImmutableArray keyValuePairs)
       : kvPairs_ = keyValuePairs;
 
 
-  V operator [](K key) {
+  V operator [](Object key) {
     // TODO(hausner): Since the keys are sorted, we could do a binary
     // search. But is it worth it?
     for (int i = 0; i < kvPairs_.length - 1; i += 2) {
       if (key == kvPairs_[i]) {
         return kvPairs_[i+1];
       }
     }
     return null;
   }
 
@@ skipping 14 matching lines @@
   }
 
   Iterable<K> get keys {
     return new _ImmutableMapKeyIterable<K>(this);
   }
 
   Iterable<V> get values {
     return new _ImmutableMapValueIterable<V>(this);
   }
 
-  bool containsKey(K key) {
+  bool containsKey(Object key) {
     for (int i = 0; i < kvPairs_.length; i += 2) {
       if (key == kvPairs_[i]) {
         return true;
       }
     }
     return false;
   }
 
-  bool containsValue(V value) {
+  bool containsValue(Object value) {
     for (int i = 1; i < kvPairs_.length; i += 2) {
       if (value == kvPairs_[i]) {
         return true;
       }
     }
     return false;
   }
 
   void operator []=(K key, V value) {
     throw new UnsupportedError("Cannot set value in unmodifiable Map");
   }
 
   V putIfAbsent(K key, V ifAbsent()) {
     throw new UnsupportedError("Cannot set value in unmodifiable Map");
   }
 
   void clear() {
     throw new UnsupportedError("Cannot clear unmodifiable Map");
   }
 
-  V remove(K key) {
+  V remove(Object key) {
     throw new UnsupportedError("Cannot remove from unmodifiable Map");
   }
 
   String toString() {
     return Maps.mapToString(this);
   }
 }
 
 class _ImmutableMapKeyIterable<E> extends IterableBase<E> {
   final ImmutableMap _map;
@@ skipping 49 matching lines @@
       _current = _map.kvPairs_[newIndex * 2 + 1];
       return true;
     }
     _current = null;
     _index = _map.length;
     return false;
   }
 
   E get current => _current;
 }