Use named constants for the mask values used to narrow byte array values.

runtime/lib/byte_array.dart

Index: runtime/lib/byte_array.dart
diff --git a/runtime/lib/byte_array.dart b/runtime/lib/byte_array.dart
index 9eac822e486b707c86cba9764d08622250e2e5db..7a807d7aa246406e893ba9b1097b01ff29fa7a36 100644
--- a/runtime/lib/byte_array.dart
+++ b/runtime/lib/byte_array.dart
@@ -164,44 +164,48 @@ class _ByteArrayBase {
     return list;
   }
 
-
   // Implementation
 
+  static final int _UINT8_MAX = (1 << 8) - 1;
+  static final int _UINT16_MAX = (1 << 16) - 1;
+  static final int _UINT32_MAX = (1 << 32) - 1;
+  static final int _UINT64_MAX = (1 << 64) - 1;
+
   int _toInt(int value, int mask) {
     int result = value & mask;
     return result > (mask >> 1) ? (result - mask) : result;
   }
 
   int _toInt8(int value) {
-    return _toInt(value, (1 << 8) - 1);  // TODO(cshapiro): use a named value
+    return _toInt(value, _UINT8_MAX);
   }
 
   int _toUint8(int value) {
-    return value & ((1 << 8) - 1);  // TODO(cshapiro): use a named value
+    return value & _UINT8_MAX;
   }
 
   int _toInt16(int value) {
-    return _toInt(value, (1 << 16) - 1);  // TODO(cshapiro): use a named value
+    return _toInt(value, _UINT16_MAX);
   }
 
   int _toUint16(int value) {
-    return value & ((1 << 16) - 1);  // TODO(cshapiro): use a named value
+    return value & _UINT16_MAX;
   }
 
   int _toInt32(int value) {
-    return _toInt(value, (1 << 32) - 1);  // TODO(cshapiro): use a named value
+    return _toInt(value, _UINT32_MAX);
   }
 
   int _toUint32(int value) {
-    return value & ((1 << 32) - 1);  // TODO(cshapiro): use a named value
+    return value & _UINT32_MAX;
   }
 
   int _toInt64(int value) {
-    return _toInt(value, (1 << 64) - 1);  // TODO(cshapiro): use a named value
+    return _toInt(value, _UINT64_MAX);
   }
 
   int _toUint64(int value) {
-    return value & ((1 << 64) - 1);  // TODO(cshapiro): use a named value
+    return value & _UINT64_MAX;
   }
 
   int _length() native "ByteArray_getLength";