Remove deprecated string features.

sdk/lib/io/mime_multipart_parser.dart

Index: sdk/lib/io/mime_multipart_parser.dart
diff --git a/sdk/lib/io/mime_multipart_parser.dart b/sdk/lib/io/mime_multipart_parser.dart
index 1ae374c2148e2b1e11fc387bc054234db71a7b01..a4ba3c0f9221428071960cdc21c218a881c57efc 100644
--- a/sdk/lib/io/mime_multipart_parser.dart
+++ b/sdk/lib/io/mime_multipart_parser.dart
@@ -41,7 +41,7 @@ class _MimeMultipartParser {
   // [boundary]. The boundary should be as specified in the content
   // type parameter, that is without the -- prefix.
   _MimeMultipartParser(String boundary) {
-    List<int> charCodes = boundary.charCodes;
+    List<int> charCodes = boundary.codeUnits;
     _boundary = new List<int>.fixedLength(4 + charCodes.length);
     // Set-up the matching boundary preceding it with CRLF and two
     // dashes.
@@ -293,9 +293,9 @@ class _MimeMultipartParser {
   }
 
   int _toLowerCase(int byte) {
-    final int aCode = "A".charCodeAt(0);
-    final int zCode = "Z".charCodeAt(0);
-    final int delta = "a".charCodeAt(0) - aCode;
+    final int aCode = "A".codeUnitAt(0);
+    final int zCode = "Z".codeUnitAt(0);
+    final int delta = "a".codeUnitAt(0) - aCode;
     return (aCode <= byte && byte <= zCode) ? byte + delta : byte;
   }