Fix error message for unexpected characters in JSON.

tests/lib/convert/json_test.dart

 // Copyright (c) 2012, 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.
 
 library json_test;
 
 import "package:expect/expect.dart";
 import "dart:convert";
 
 bool badFormat(e) => e is FormatException;
@@ skipping 16 matching lines @@
     } else if (expected is num) {
       Expect.equals(expected is int, actual is int, "$path: same number type");
       Expect.isTrue(expected.compareTo(actual) == 0,
                     "$path: $expected vs. $actual");
     } else {
       // String, bool, null.
       Expect.equals(expected, actual, path);
     }
   }
   for (var reviver in [null, (k, v) => v]) {
+    var name = (reviver == null) ? "" : "reviver:";
     var value = JSON.decode(json, reviver: reviver);
-    compare(expected, value, "$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}");
   }
 }
 
 String escape(String s) {
   var sb = new StringBuffer();
   for (int i = 0; i < s.length; i++) {
     int code = s.codeUnitAt(i);
     if (code == '\\'.codeUnitAt(0)) sb.write(r'\\');
     else if (code == '\"'.codeUnitAt(0)) sb.write(r'\"');
     else if (code >= 32 && code < 127) sb.writeCharCode(code);
@@ skipping 232 matching lines @@
 }
 
 main() {
   testNumbers();
   testStrings();
   testWords();
   testObjects();
   testArrays();
   testWhitespace();
 }