Fix modification of map while iterating in SimpleJsonFormat

pkg/serialization/lib/src/format.dart

Index: pkg/serialization/lib/src/format.dart
diff --git a/pkg/serialization/lib/src/format.dart b/pkg/serialization/lib/src/format.dart
index ab602c92a2b80c29997324b736f7a592c07b1f08..a1726f0f5b979a82627cc96c356a793509399ba9 100644
--- a/pkg/serialization/lib/src/format.dart
+++ b/pkg/serialization/lib/src/format.dart
@@ -154,9 +154,15 @@ class SimpleJsonFormat extends Format {
    * 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);
   }
 
   /**