Add optimized _OneByteString.toLowerCase.

sdk/lib/io/common.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 dart.io;
 
 // Constants used when working with native ports.
 // These must match the constants in runtime/bin/dartutils.h class CObject.
 const int _SUCCESS_RESPONSE = 0;
 const int _ILLEGAL_ARGUMENT_RESPONSE = 1;
@@ skipping 104 matching lines @@
     newBuffer[i] = value;
     j++;
   }
   return new _BufferAndStart(newBuffer, 0);
 }
 
 
 class _IOCrypto {
   external static Uint8List getRandomBytes(int count);
 }
+
+class _ASCII {
+  static int toLowerCaseByte(int x) {
+    // Optimzed version:
+    //  -  0x41 is 'A'

[kevmoo, 2014/03/25 14:38:25]

LOVE the explicit comments.

No guessing. Thanks!

[Anders Johnsen, 2014/03/25 15:00:22]

Yeah, it helps understanding. Moved stuff around, PTAL :)

+    //  -  0x7f is ASCII mask
+    //  -  26 is the number of alpha characters.
+    //  -  0x20 is the delta between lower and upper chars.
+    return ((x - 0x41) & 0x7f) < 26 ? x | 0x20 : x;
+  }
+
+  static String toLowerCase(String str) {
+    final length = str.length;
+    List result = new Uint8List(length);
+    for (int i = 0; i < length; i++) {
+      int codeUnit = str.codeUnitAt(i);
+      if (codeUnit > 127) {
+        throw new FormatException("ASCII string contains non-ASCII character.");
+      }
+      result[i] = toLowerCaseByte(codeUnit);
+    }
+    return new String.fromCharCodes(result);
+  }
+}