Use a re-usable buffer for writing HTTP headers. This includes a new header limit of 8192 bytes.

sdk/lib/io/http_headers.dart

 // Copyright (c) 2013, 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;
 
 class _HttpHeaders implements HttpHeaders {
   _HttpHeaders(String this.protocolVersion)
       : _headers = new Map<String, List<String>>();
 
@@ skipping 326 matching lines @@
         protocolVersion == "1.1") {
       contentLength = -1;
     }
   }
 
   void _finalize() {
     _synchronize();
     _mutable = false;
   }
 
-  _write(BytesBuilder builder) {
-    final COLONSP = const [_CharCode.COLON, _CharCode.SP];
-    final COMMASP = const [_CharCode.COMMA, _CharCode.SP];
-    final CRLF = const [_CharCode.CR, _CharCode.LF];
+  int _write(Uint8List buffer, int offset) {
+    void write(List<int> bytes) {
+      int len = bytes.length;
+      for (int i = 0; i < len; i++) {
+        buffer[offset + i] = bytes[i];
+      }
+      offset += len;
+    }
 
     // Format headers.
     _headers.forEach((String name, List<String> values) {
       bool fold = _foldHeader(name);
       var nameData = name.codeUnits;
-      builder.add(nameData);
-      builder.add(const [_CharCode.COLON, _CharCode.SP]);
+      write(nameData);
+      write(const [_CharCode.COLON, _CharCode.SP]);
       for (int i = 0; i < values.length; i++) {
         if (i > 0) {
           if (fold) {
-            builder.add(const [_CharCode.COMMA, _CharCode.SP]);
+            write(const [_CharCode.COMMA, _CharCode.SP]);
           } else {
-            builder.add(const [_CharCode.CR, _CharCode.LF]);
-            builder.add(nameData);
-            builder.add(const [_CharCode.COLON, _CharCode.SP]);
+            write(const [_CharCode.CR, _CharCode.LF]);
+            write(nameData);
+            write(const [_CharCode.COLON, _CharCode.SP]);
           }
         }
-        builder.add(values[i].codeUnits);
+        write(values[i].codeUnits);
       }
-      builder.add(const [_CharCode.CR, _CharCode.LF]);
+      write(const [_CharCode.CR, _CharCode.LF]);
     });
+    return offset;
   }
 
   String toString() {
     StringBuffer sb = new StringBuffer();
     _headers.forEach((String name, List<String> values) {
       sb.write(name);
       sb.write(": ");
       bool fold = _foldHeader(name);
       for (int i = 0; i < values.length; i++) {
         if (i > 0) {
@@ skipping 440 matching lines @@
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
   void forEach(void f(K key, V value)) => _map.forEach(f);
   Iterable<K> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }