Fix error message for unexpected characters in JSON.

tests/lib/convert/json_test.dart

Index: tests/lib/convert/json_test.dart
diff --git a/tests/lib/convert/json_test.dart b/tests/lib/convert/json_test.dart
index 2d3e084e139b80f3e5560675ed142e1f2bb4b76a..e1bc6b29776ed97277c1c09f3aef48392a5348e7 100644
--- a/tests/lib/convert/json_test.dart
+++ b/tests/lib/convert/json_test.dart
@@ -34,14 +34,15 @@ void testJson(json, expected) {
     }
   }
   for (var reviver in [null, (k, v) => v]) {
+    var name = (reviver == null) ? "" : "reviver:";
     var value = JSON.decode(json, reviver: reviver);
-    compare(expected, value, "$value");
+    compare(expected, value, "$name$value");
     value = JSON.decode(" $json ", reviver: reviver);
-    compare(expected, value, "-$value-");
+    compare(expected, value, "$name-$value-");
     value = JSON.decode("[$json]", reviver: reviver);
-    compare([expected], value, "[$value]");
+    compare([expected], value, "$name[$value]");
     value = JSON.decode('{"x":$json}', reviver: reviver);
-    compare({"x":expected}, value, "{x:$value}");
+    compare({"x":expected}, value, "$name{x:$value}");
   }
 }