Re-apply "Add a non-dart:io WebSocket implementation to http_parser."

pkg/http_parser/lib/src/bytes_builder.dart

Index: pkg/http_parser/lib/src/bytes_builder.dart
diff --git a/sdk/lib/io/bytes_builder.dart b/pkg/http_parser/lib/src/bytes_builder.dart
similarity index 93%
copy from sdk/lib/io/bytes_builder.dart
copy to pkg/http_parser/lib/src/bytes_builder.dart
index 610c50a74638e5c8d1ab3844f8522c16814ae9a3..446e1751baa4aeaa8978235878eee3b979f932d8 100644
--- a/sdk/lib/io/bytes_builder.dart
+++ b/pkg/http_parser/lib/src/bytes_builder.dart
@@ -2,7 +2,15 @@
 // 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;
+// This is a copy of "dart:io"'s BytesBuilder implementation, from
+// sdk/lib/io/bytes_builder.dart. It's copied here to make it available to
+// non-"dart:io" applications (issue 18348).
+//
+// Because it's copied directly, there are no modifications from the original.
+library http_parser.bytes_builder;
+
+import 'dart:math';
+import 'dart:typed_data';
 
 /**
  * Builds a list of bytes, allowing bytes and lists of bytes to be added at the