Make HTTP headers use a growing buffer, not a fixed-size 8K one.

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 {
   final Map<String, List<String>> _headers;
   final String protocolVersion;
 
@@ skipping 447 matching lines @@
          _noFoldingHeaders.indexOf(name) != -1)) {
       return false;
     }
     return true;
   }
 
   void _finalize() {
     _mutable = false;
   }
 
+  void _build(BytesBuilder builder) {
+    for (String name in _headers.keys) {
+      List<String> values = _headers[name];
+      bool fold = _foldHeader(name);
+      var nameData = name.codeUnits;
+      builder.add(nameData);
+      builder.addByte(_CharCode.COLON);
+      builder.addByte(_CharCode.SP);
+      for (int i = 0; i < values.length; i++) {
+        if (i > 0) {
+          if (fold) {
+            builder.addByte(_CharCode.COMMA);
+            builder.addByte(_CharCode.SP);
+          } else {
+            builder.addByte(_CharCode.CR);
+            builder.addByte(_CharCode.LF);
+            builder.add(nameData);
+            builder.addByte(_CharCode.COLON);
+            builder.addByte(_CharCode.SP);
+          }
+        }
+        builder.add(values[i].codeUnits);
+      }
+      builder.addByte(_CharCode.CR);
+      builder.addByte(_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.
@@ skipping 512 matching lines @@
             (codeUnit >= 0x23 && codeUnit <= 0x2B) ||
             (codeUnit >= 0x2D && codeUnit <= 0x3A) ||
             (codeUnit >= 0x3C && codeUnit <= 0x5B) ||
             (codeUnit >= 0x5D && codeUnit <= 0x7E))) {
         throw new FormatException(
             "Invalid character in cookie value, code unit: '$codeUnit'");
       }
     }
   }
 }