use proper notation for true, false, and null in doc comments

pkg/fixnum/lib/src/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.
 
 part of fixnum;
 
 /**
  * An immutable 64-bit signed integer, in the range [-2^63, 2^63 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
@@ skipping 422 matching lines @@
     } else {
       res2 = 0;
       res1 = 0;
       res0 = a2 >> (n - _BITS01);
     }
 
     return Int64._masked(res0, res1, res2);
   }
 
   /**
-   * Returns [true] if this [Int64] has the same numeric value as the
+   * Returns [:true:] if this [Int64] has the same numeric value as the
    * given object.  The argument may be an [int] or an [IntX].
    */
   bool operator ==(other) {
     Int64 o;
     if (other is Int64) {
       o = other;
     } else if (other is int) {
       if (_h == 0 && _m == 0) return _l == other;
       // Since we know one of [_h] or [_m] is non-zero, if [other] fits in the
       // low word then it can't be numerically equal.
@@ skipping 579 matching lines @@
       if (r0 == 0 && r1 == 0 && r2 == 0) {
         return ZERO;
       } else {
         return _sub(b0, b1, b2, r0, r1, r2);
       }
     } else {
       return _negate(r0, r1, r2);
     }
   }
 }