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

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 252 matching lines @@
     int value;
     if (_i >= 0) {
       value = _i >> n;
     } else {
       value = (_i >> n) & ((1 << (32 - n)) - 1);
     }
     return new Int32(value);
   }
 
   /**
-   * Returns [true] if this [Int32] has the same numeric value as the
+   * Returns [:true:] if this [Int32] has the same numeric value as the
    * given object.  The argument may be an [int] or an [IntX].
    */
   bool operator ==(other) {
     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;
     }
@@ skipping 84 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);
 }