Fix assert in int64 and add test

pkg/fixnum/test/int_64_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 int64test;
 import 'package:fixnum/fixnum.dart';
 import 'package:unittest/unittest.dart';
 
 void main() {
   group("arithmetic operators", () {
@@ skipping 544 matching lines @@
         expect(Int64.parseRadix(s, r).toString(), x);
       }
       check('ghoul', 36, '27699213');
       check('ghoul', 35, '24769346');
       // Min and max value.
       check("-9223372036854775808", 10, "-9223372036854775808");
       check("9223372036854775807", 10, "9223372036854775807");
       // Overflow during parsing.
       check("9223372036854775808", 10, "-9223372036854775808");
     });
+
+    test("parseRadixN", () {
+      check(String s, int r) {
+        expect(Int64.parseRadix(s, r).toRadixString(r), s);
+      }
+      check("2ppp111222333", 33);  // This value & radix requires three chunks.
+    });
   });
 
   group("string representation", () {
     test("toString", () {
       expect(new Int64.fromInt(0).toString(), "0");
       expect(new Int64.fromInt(1).toString(), "1");
       expect(new Int64.fromInt(-1).toString(), "-1");
       expect(new Int64.fromInt(-10).toString(), "-10");
       expect(Int64.MIN_VALUE.toString(), "-9223372036854775808");
       expect(Int64.MAX_VALUE.toString(), "9223372036854775807");
@@ skipping 50 matching lines @@
       expect(Int64.MAX_VALUE.toRadixString(10), "9223372036854775807");
       expect(Int64.MAX_VALUE.toRadixString(11), "1728002635214590697");
       expect(Int64.MAX_VALUE.toRadixString(12), "41a792678515120367");
       expect(Int64.MAX_VALUE.toRadixString(13), "10b269549075433c37");
       expect(Int64.MAX_VALUE.toRadixString(14), "4340724c6c71dc7a7");
       expect(Int64.MAX_VALUE.toRadixString(15), "160e2ad3246366807");
       expect(Int64.MAX_VALUE.toRadixString(16), "7fffffffffffffff");
     });
   });
 }