Remove support for operator negate and replace occurrences with unary

pkg/fixnum/int64.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.
 
 /**
  * An immutable 64-bit signed integer, in the range [-2^63, 2^63 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
 class int64 implements intx {
 
@@ skipping 255 matching lines @@
     int64 o = _promote(other);
 
     int sum0 = _l - o._l;
     int sum1 = _m - o._m + _shiftRight(sum0, _BITS);
     int sum2 = _h - o._h + _shiftRight(sum1, _BITS);
 
     int64 result = new int64._bits(sum0 & _MASK, sum1 & _MASK, sum2 & _MASK_2);
     return result;
   }
 
-  int64 operator negate() {
+  int64 operator -() {
     // Like 0 - this.
     int sum0 = -_l;
     int sum1 = -_m + _shiftRight(sum0, _BITS);
     int sum2 = -_h + _shiftRight(sum1, _BITS);
 
     return new int64._bits(sum0 & _MASK, sum1 & _MASK, sum2 & _MASK_2);
   }
 
   int64 operator *(other) {
     int64 o = _promote(other);
@@ skipping 799 matching lines @@
         }
       }
       return ZERO;
     }
 
     // Generate the quotient using bit-at-a-time long division.
     return _divModHelper(aIsCopy ? a : new int64._copy(a), b, negative,
         aIsNegative, aIsMinValue, computeRemainder);
   }
 }