In JsonEncoder, test if map has only string keys, and use toEncodable if not.

sdk/lib/convert/json.dart

 // Copyright (c) 2013, 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.
 
 part of dart.convert;
 
 /**
  * Error thrown by JSON serialization if an object cannot be serialized.
  *
  * The [unsupportedObject] field holds that object that failed to be serialized.
@@ skipping 675 matching lines @@
       writeStringContent(object);
       writeString('"');
       return true;
     } else if (object is List) {
       _checkCycle(object);
       writeList(object);
       _removeSeen(object);
       return true;
     } else if (object is Map) {
       _checkCycle(object);
-      writeMap(object);
+      // writeMap can fail if keys are not all strings.
+      var result = writeMap(object);

[Søren Gjesse, 2015/05/26 14:54:00]

result -> success

       _removeSeen(object);
-      return true;
+      return result;
     } else {
       return false;
     }
   }
 
   /** Serializes a [List]. */
   void writeList(List list) {
     writeString('[');
     if (list.length > 0) {
       writeObject(list[0]);
       for (int i = 1; i < list.length; i++) {
         writeString(',');
         writeObject(list[i]);
       }
     }
     writeString(']');
   }
 
   /** Serializes a [Map]. */
-  void writeMap(Map<String, Object> map) {
+  bool writeMap(Map<String, Object> map) {
+    List keyValueList = new List(map.length * 2);
+    int i = 0;
+    bool allStringKeys = true;
+    map.forEach((key, value) {
+      if (key is! String) {
+        allStringKeys = false;
+      }
+      keyValueList[i++] = key;
+      keyValueList[i++] = value;
+    });
+    if (!allStringKeys) return false;
     writeString('{');
     String separator = '"';
-    map.forEach((String key, value) {
+    for (int i = 0; i < keyValueList.length; i += 2) {
       writeString(separator);
       separator = ',"';
-      writeStringContent(key);
+      writeStringContent(keyValueList[i]);
       writeString('":');
-      writeObject(value);
-    });
+      writeObject(keyValueList[i + 1]);
+    }
     writeString('}');
+    return true;
   }
 }
 
 /**
  * A modification of [_JsonStringifier] which indents the contents of [List] and
  * [Map] objects using the specified indent value.
  *
  * Subclasses should implement [writeIndentation].
  */
 abstract class _JsonPrettyPrintMixin implements _JsonStringifier {
@@ skipping 290 matching lines @@
         buffer.setRange(index, end, indent);
         index = end;
       } else {
         for (int i = 0; i < indentLength; i++) {
           writeByte(indent[i]);
         }
       }
     }
   }
 }