Improve _toInt64() and _toUint64() conversions in TypedData.

runtime/lib/typed_data.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.
 
 // patch classes for Int8List ..... Float64List and ByteData implementations.
 
 import "dart:_internal";
 import 'dart:math' show Random;
 
 patch class Int8List {
@@ skipping 3570 matching lines @@
   return _toInt(value, 0xFFFFFFFF);
 }
 
 
 int _toUint32(int value) {
   return value & 0xFFFFFFFF;
 }
 
 
 int _toInt64(int value) {
-  return _toInt(value, 0xFFFFFFFFFFFFFFFF);  // TODO(regis): Avoid bigint mask.
+  // Avoid bigint mask when possible.
+  return (ClassID.getID(value) == ClassID.cidBigint) ?
+      _toInt(value, 0xFFFFFFFFFFFFFFFF) : value;
 }
 
 
 int _toUint64(int value) {
-  return value & 0xFFFFFFFFFFFFFFFF;  // TODO(regis): Avoid bigint mask.
+  // Avoid bigint mask when possible.
+  return (ClassID.getID(value) == ClassID.cidBigint) ?
+      _toInt(value, 0xFFFFFFFFFFFFFFFF) : value;
 }
 
 
 void _rangeCheck(int listLength, int start, int length) {
   if (length < 0) {
     throw new RangeError.value(length);
   }
   if (start < 0) {
     throw new RangeError.value(start);
   }
@@ skipping 16 matching lines @@
     return value;
   }
   return object;
 }
 
 
 _newRangeError(int index, int length) {
   String message = "$index must be in the range [0..$length)";
   return new RangeError(message);
 }