Cleanup IntX to implement Comparable<FixNum> which is more descriptive than Comparable as it would …

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.
  */
@@ skipping 266 matching lines @@
     if (other is Int32) {
       return _i == other._i;
     } else if (other is Int64) {
       return this.toInt64() == other;
     } else if (other is int) {
       return _i == other;
     }
     return false;
   }
 
-  int compareTo(Comparable other) {
+  int compareTo(IntX other) {
     if (other is Int64) {
       return this.toInt64().compareTo(other);
     }
     return _i.compareTo(_toInt(other));
   }
 
   bool operator <(other) {
     if (other is Int64) {
       return this.toInt64() < other;
     }
@@ skipping 70 matching lines @@
 
   double toDouble() => _i.toDouble();
   int toInt() => _i;
   Int32 toInt32() => this;
   Int64 toInt64() => new Int64(_i);
 
   String toString() => _i.toString();
   String toHexString() => _i.toRadixString(16);
   String toRadixString(int radix) => _i.toRadixString(radix);
 }