Fix modification of map while iterating in SimpleJsonFormat

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 136 matching lines @@
         if (storeRoundTripInfo) each[RULE] = rule.number;
       }
     }
   }
 
   /**
    * For one particular entry, which is either a Map or a List, update it
    * to turn References into a nested List/Map.
    */
   jsonifyEntry(map, Writer w) {
+    // Note, if this is a Map, and the key might be a reference, we need to
+    // bend over backwards to avoid concurrent modifications. Non-string keys
+    // won't actually work if we try to write this to json, but might happen
+    // if e.g. sending between isolates.
+    var updates = new Map();
     keysAndValues(map).forEach((key, value) {
-      if (value is Reference) map[key] = w.stateForReference(value);
+      if (value is Reference) updates[key] = w.stateForReference(value);
     });
+    updates.forEach((k, v) => map[k] = v);
   }
 
   /**
    * Read serialized data saved in this format, which should look like
    * either a simple type, a List or a Map and return the Map
    * representation that the reader expects, with top-level
    * entries for "rules", "data", and "roots". Nested lists/maps will be
    * converted into Reference objects. Note that if the data was not written
    * with [storeRoundTripInfo] true this will fail.
    */
@@ skipping 286 matching lines @@
       return new Reference(r, a, b);
     }
   }
 
   /** Return the next element from the input. */
   _next(Iterator input) {
     input.moveNext();
     return input.current;
   }
 }