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

sdk/lib/convert/json.dart

Index: sdk/lib/convert/json.dart
diff --git a/sdk/lib/convert/json.dart b/sdk/lib/convert/json.dart
index fa0886d7f3b535a654ac7b02fc43bee83c73137f..7d5f929fdda5c49a8986b761637d88cd852c11a9 100644
--- a/sdk/lib/convert/json.dart
+++ b/sdk/lib/convert/json.dart
@@ -693,9 +693,10 @@ abstract class _JsonStringifier {
       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;
     }
@@ -715,17 +716,29 @@ abstract class _JsonStringifier {
   }
 
   /** 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;
   }
 }