Add comments for constants which replaced large integer literals

runtime/lib/typed_data_patch.dart

 // Copyright (c) 2013, 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.
 
 import "dart:_internal" hide Symbol;
 import "dart:collection" show ListBase;
 import 'dart:math' show Random;
 
 @patch
 class ByteData implements TypedData {
@@ skipping 3026 matching lines @@
 }
 
 int _toInt32(int value) {
   return _toInt(value, 0xFFFFFFFF);
 }
 
 int _toUint32(int value) {
   return value & 0xFFFFFFFF;
 }
 
+// Note: in --limit-ints-to-64-bits mode all integers are 64-bit already.
+// Still, it is harmless to apply _uint64Mask because (1 << 64) is 0 (all bits
+// are shifted out), so _uint64Mask is -1 (its bit pattern is 0xffffffffffffffff).
 const _uint64Mask = (1 << 64) - 1;
 
 int _toInt64(int value) {
   // Avoid bigint mask when possible.
   return (ClassID.getID(value) == ClassID.cidBigint)
       ? _toInt(value, _uint64Mask)
       : value;
 }
 
 int _toUint64(int value) {
@@ skipping 21 matching lines @@
         'BYTES_PER_ELEMENT ($alignment)');
   }
 }
 
 int _defaultIfNull(object, value) {
   if (object == null) {
     return value;
   }
   return object;
 }