Add optimized _OneByteString.toLowerCase.

sdk/lib/io/common.dart

Index: sdk/lib/io/common.dart
diff --git a/sdk/lib/io/common.dart b/sdk/lib/io/common.dart
index ea355801147789b965aafbea16516674ce4680f8..9d9a81826ec1b706c9ee9ea4d252093f3a9cc263 100644
--- a/sdk/lib/io/common.dart
+++ b/sdk/lib/io/common.dart
@@ -122,3 +122,27 @@ _BufferAndStart _ensureFastAndSerializableByteData(
 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);
+  }
+}