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

sdk/lib/io/http_headers.dart

Index: sdk/lib/io/http_headers.dart
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index 3769b3f0d04949bb1aa6e1ce5bbfee041ef9aa21..b3e4854333905ae78fc36f55d42d67b3449b64ef 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -465,6 +465,34 @@ class _HttpHeaders implements HttpHeaders {
     _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;