Revert more changes

Author: osa1

pkgs/fixnum/lib/src/intx.dart

@@ -6,7 +6,7 @@ import 'int32.dart';
 import 'int64.dart';
 
 /// A fixed-precision integer.
-abstract class IntX implements Comparable<Object> {
+abstract class IntX implements Comparable<IntX> {
   /// Addition operator.
   IntX operator +(Object other);
 

pkgs/fixnum/lib/src/utilities.dart

@@ -16,12 +16,12 @@ int validateRadix(int radix) =>
 /// not a valid digit in any radix in the range 2 through 36.
 int decodeDigit(int c) {
   // Hex digit char codes
-  const int c0 = 48; // '0'.codeUnitAt(0)
-  const int ca = 97; // 'a'.codeUnitAt(0)
+  const c0 = 48; // '0'.codeUnitAt(0)
+  const ca = 97; // 'a'.codeUnitAt(0)
 
-  int digit = c ^ c0;
+  var digit = c ^ c0;
   if (digit < 10) return digit;
-  int letter = (c | 0x20) - ca;
+  var letter = (c | 0x20) - ca;
   if (letter >= 0) {
     // Returns values above 36 for invalid digits.
     // The value is checked against the actual radix where the return