Make JSON encoder take extra function argument to use instead of toJson calls.

pkg/json/lib/json.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.
 
 /**
  * Utilities for encoding and decoding JSON (JavaScript Object Notation) data.
  */
 
 library json;
 
+import "dart:collection" show HashSet;
+
 // JSON parsing and serialization.
 
 /**
  * Error thrown by JSON serialization if an object cannot be serialized.
  *
  * The [unsupportedObject] field holds that object that failed to be serialized.
  *
  * If an object isn't directly serializable, the serializer calls the 'toJson'
  * method on the object. If that call fails, the error will be stored in the
  * [cause] field. If the call returns an object that isn't directly
@@ skipping 637 matching lines @@
       slice = "'${source.substring(position)}'";
     } else {
       slice = "'${source.substring(position, sliceEnd)}...'";
     }
     throw new FormatException("Unexpected character at $position: $slice");
   }
 }
 
 
 class _JsonStringifier {
-  StringSink sink;
-  List<Object> seen;  // TODO: that should be identity set.
+  final StringSink sink;
+  final Set<Object> seen;
 
-  _JsonStringifier(this.sink) : seen = [];
+  _JsonStringifier(this.sink) : seen = new HashSet.identity();
 
   static String stringify(final object) {
     StringBuffer output = new StringBuffer();
     _JsonStringifier stringifier = new _JsonStringifier(output);
     stringifier.stringifyValue(object);
     return output.toString();
   }
 
   static void printOn(final object, StringSink output) {
     _JsonStringifier stringifier = new _JsonStringifier(output);
@@ skipping 46 matching lines @@
         charCodes.add(JsonParser.BACKSLASH);
         charCodes.add(charCode);
       } else {
         charCodes.add(charCode);
       }
     }
     sb.write(needsEscape ? new String.fromCharCodes(charCodes) : s);
   }
 
   void checkCycle(final object) {
-    // TODO: use Iterables.
-    for (int i = 0; i < seen.length; i++) {
-      if (identical(seen[i], object)) {
-        throw new JsonCyclicError(object);
-      }
+    if (seen.contains(object)) {
+      throw new JsonCyclicError(object);
     }
     seen.add(object);
   }
 
   void stringifyValue(final object) {
     // Tries stringifying object directly. If it's not a simple value, List or
     // Map, call toJson() to get a custom representation and try serializing
     // that.
     if (!stringifyJsonValue(object)) {
       checkCycle(object);
       try {
         var customJson = object.toJson();
         if (!stringifyJsonValue(customJson)) {
           throw new JsonUnsupportedObjectError(object);
         }
-        seen.removeLast();
+        seen.remove(object);
       } catch (e) {
         throw new JsonUnsupportedObjectError(object, cause: e);
       }
     }
   }
 
   /**
    * Serializes a [num], [String], [bool], [Null], [List] or [Map] value.
    *
    * Returns true if the value is one of these types, and false if not.
@@ skipping 24 matching lines @@
       sink.write('[');
       if (a.length > 0) {
         stringifyValue(a[0]);
         // TODO: switch to Iterables.
         for (int i = 1; i < a.length; i++) {
           sink.write(',');
           stringifyValue(a[i]);
         }
       }
       sink.write(']');
-      seen.removeLast();
+      seen.remove(object);
       return true;
     } else if (object is Map) {
       checkCycle(object);
       Map<String, Object> m = object;
       sink.write('{');
       bool first = true;
       m.forEach((String key, Object value) {
         if (!first) {
           sink.write(',"');
         } else {
           sink.write('"');
         }
         escape(sink, key);
         sink.write('":');
         stringifyValue(value);
         first = false;
       });
       sink.write('}');
-      seen.removeLast();
+      seen.remove(object);
       return true;
     } else {
       return false;
     }
   }
 }