Remove unmodifiable map wrappers that could instead use UnmodifiableMapView.

pkg/yaml/lib/src/yaml_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.
 
 library yaml_map;
 
 import 'deep_equals.dart';
 import 'utils.dart';
 
 /// This class wraps behaves almost identically to the normal Dart Map
@@ skipping 37 matching lines @@
   int get hashCode => hashCodeFor(_map);
 
   bool operator ==(other) {
     if (other is! YamlMap) return false;
     return deepEquals(this, other);
   }
 
   /// Wraps an object for use as a key in the map.
   _wrapKey(obj) {
     if (obj != null && obj is! bool && obj is! List &&
-        (obj is! double || !obj.isNan()) &&
+        (obj is! double || !obj.isNan) &&
         (obj is! Map || obj is YamlMap)) {
       return obj;
     } else if (obj is Map) {
       return new YamlMap._wrap(obj);
     }
     return new _WrappedHashKey(obj);
   }
 
   /// Unwraps an object that was used as a key in the map.
   _unwrapKey(obj) => obj is _WrappedHashKey ? obj.value : obj;
 }
 
 /// A class for wrapping normally-unhashable objects that are being used as keys
 /// in a YamlMap.
 class _WrappedHashKey {
   final value;
 
   _WrappedHashKey(this.value);
 
   int get hashCode => hashCodeFor(value);
 
   String toString() => value.toString();
 
   /// This is defined as both values being structurally equal.
   bool operator ==(other) {
     if (other is! _WrappedHashKey) return false;
     return deepEquals(this.value, other.value);
   }
 }