Migrate fixnum tests to unittest. Fix int32 rollover bug.

pkg/fixnum/lib/src/int32.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.
 
 part of fixnum;
 
 /**
  * An immutable 32-bit signed integer, in the range [-2^31, 2^31 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
 class int32 implements intx {
 
   /**
    * The maximum positive value attainable by an [int32], namely
    * 2147483647.
    */
   static const int32 MAX_VALUE = const int32._internal(0x7FFFFFFF);
 
   /**
    * The minimum positive value attainable by an [int32], namely
    * -2147483648.
    */
-  static int32 MIN_VALUE = const int32._internal(0x80000000);
+  static int32 MIN_VALUE = const int32._internal(-0x80000000);
 
   /**
    * An [int32] constant equal to 0.
    */
   static int32 ZERO = const int32._internal(0);
 
   /**
    * An [int32] constant equal to 1.
    */
   static int32 ONE = const int32._internal(1);
@@ skipping 313 matching lines @@
   }
 
   int toInt() => _i;
   int32 toInt32() => this;
   int64 toInt64() => new int64.fromInt(_i);
 
   String toString() => _i.toString();
   String toHexString() => _i.toRadixString(16);
   String toRadixString(int radix) => _i.toRadixString(radix);
 }