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

tests/lib/convert/json_toEncodable_reviver_test.dart

Index: tests/lib/convert/json_toEncodable_reviver_test.dart
diff --git a/tests/lib/convert/json_toEncodable_reviver_test.dart b/tests/lib/convert/json_toEncodable_reviver_test.dart
index 58438875832ba8af0ec99feb114e2914d174648b..d3d507d45182fe5ade55013ef9a21f93dcad5f84 100644
--- a/tests/lib/convert/json_toEncodable_reviver_test.dart
+++ b/tests/lib/convert/json_toEncodable_reviver_test.dart
@@ -38,4 +38,14 @@ main() {
   var a = extendedJson.decode(extendedJson.encode(new A(499)));
   Expect.isTrue(a is A);
   Expect.equals(499, a.x);
+
+  testInvalidMap();
+}
+
+
+void testInvalidMap() {
+  var map = {"a" : 42, "b": 42, 37: 42};  // Non-string key.
+  var enc = new JsonEncoder((x) => "fixed");
+  var res = enc.convert(map);
+  Expect.equals('"fixed"', res);
 }