Add optimized _OneByteString.toLowerCase.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 207c08d104d4d423ffcc6ea8c197f211c02aa1cc..8dd0bae65df1fa075afad9a95d8c9647449f1e9e 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -364,7 +364,7 @@ class _HttpClientResponse
       // Digest authentication only supports the MD5 algorithm.
       if (cr.scheme == _AuthenticationScheme.DIGEST &&
           (header.parameters["algorithm"] == null ||
-           header.parameters["algorithm"].toLowerCase() == "md5")) {
+           _ASCII.toLowerCase(header.parameters["algorithm"]) == "md5")) {
         if (cr.nonce == null || cr.nonce == header.parameters["nonce"]) {
           // If the nonce is not set then this is the first authenticate
           // response for these credentials. Set up authentication state.
@@ -377,7 +377,7 @@ class _HttpClientResponse
           // Credentials where found, prepare for retrying the request.
           return retry();
         } else if (header.parameters["stale"] != null &&
-                   header.parameters["stale"].toLowerCase() == "true") {
+                   _ASCII.toLowerCase(header.parameters["stale"]) == "true") {
           // If stale is true retry with new nonce.
           cr.nonce = header.parameters["nonce"];
           // Credentials where found, prepare for retrying the request.
@@ -926,7 +926,8 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
         if (acceptEncodings != null &&
             acceptEncodings
                 .expand((list) => list.split(","))
-                .any((encoding) => encoding.trim().toLowerCase() == "gzip") &&
+                .any((encoding) =>
+                    _ASCII.toLowerCase(encoding.trim()) == "gzip") &&
             contentEncoding == null) {
           outbound.headers.set(HttpHeaders.CONTENT_ENCODING, "gzip");
           gzip = true;
@@ -2349,8 +2350,9 @@ class _AuthenticationScheme {
   const _AuthenticationScheme(this._scheme);
 
   factory _AuthenticationScheme.fromString(String scheme) {
-    if (scheme.toLowerCase() == "basic") return BASIC;
-    if (scheme.toLowerCase() == "digest") return DIGEST;
+    scheme = _ASCII.toLowerCase(scheme);
+    if (scheme == "basic") return BASIC;
+    if (scheme == "digest") return DIGEST;
     return UNKNOWN;
   }