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..40df48a3c2fd8f739265621310daad4a7094401c 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -465,42 +465,32 @@ class _HttpHeaders implements HttpHeaders {
     _mutable = false;
   }
 
-  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.
+  void _build(BytesBuilder builder) {
     for (String name in _headers.keys) {
       List<String> values = _headers[name];
       bool fold = _foldHeader(name);
       var nameData = name.codeUnits;
-      write(nameData);
-      buffer[offset++] = _CharCode.COLON;
-      buffer[offset++] = _CharCode.SP;
+      builder.add(nameData);
+      builder.addByte(_CharCode.COLON);
+      builder.addByte(_CharCode.SP);
       for (int i = 0; i < values.length; i++) {
         if (i > 0) {
           if (fold) {
-            buffer[offset++] = _CharCode.COMMA;
-            buffer[offset++] = _CharCode.SP;
+            builder.addByte(_CharCode.COMMA);
+            builder.addByte(_CharCode.SP);
           } else {
-            buffer[offset++] = _CharCode.CR;
-            buffer[offset++] = _CharCode.LF;
-            write(nameData);
-            buffer[offset++] = _CharCode.COLON;
-            buffer[offset++] = _CharCode.SP;
+            builder.addByte(_CharCode.CR);
+            builder.addByte(_CharCode.LF);
+            builder.add(nameData);
+            builder.addByte(_CharCode.COLON);
+            builder.addByte(_CharCode.SP);
           }
         }
-        write(values[i].codeUnits);
+        builder.add(values[i].codeUnits);
       }
-      buffer[offset++] = _CharCode.CR;
-      buffer[offset++] = _CharCode.LF;
+      builder.addByte(_CharCode.CR);
+      builder.addByte(_CharCode.LF);
     }
-    return offset;
   }
 
   String toString() {