Revert "Remove deprecated mappedBy."

pkg/serialization/lib/src/format.dart

 part of serialization;
 
 /**
  * An abstract class for serialization formats. Subclasses define how data
  * is read or written to a particular output mechanism.
  */
 abstract class Format {
   /**
    * Return true if this format stores primitives in their own area and uses
    * references to them (e.g. [SimpleFlatFormat]) and false if primitives
@@ skipping 198 matching lines @@
     // the library will be ok, but we need to get rid of the extra key before
     // the data is shown to the user, so we destructively modify.
     if (data is List) {
       ruleNumber = data.last;
       data = data.take(data.length -1);
     } else if (data is Map) {
       ruleNumber = data.remove(RULE);
     } else {
       throw new SerializationException("Invalid data format");
     }
-    // Do not use map or other lazy operations for this. They do not play
+    // Do not use mappedBy or other lazy operations for this. They do not play
     // well with a function that destructively modifies its arguments.
     var newData = mapValues(data, (each) => recursivelyFixUp(each, r, result));
     result[ruleNumber].add(newData);
     return new Reference(r, ruleNumber, result[ruleNumber].length - 1);
   }
 }
 
 /**
  * Writes to a simple mostly-flat format. Details are subject to change.
  * Right now this produces a List containing null, num, and String. This is
@@ skipping 232 matching lines @@
       return new Reference(r, a, b);
     }
   }
 
   /** Return the next element from the input. */
   _next(Iterator input) {
     input.moveNext();
     return input.current;
   }
 }