Slightly speed up http-parser and http-header-writing.

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 0be5c080d6d68dfb09c2e29e4a6b2845a32b4065..85706d16d23f53e12a46ca3b939639d6e7e3e7c2 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -396,25 +396,31 @@ class _HttpHeaders implements HttpHeaders {
     }
 
     // Format headers.
-    _headers.forEach((String name, List<String> values) {
+    for (String name in _headers.keys) {
+      List<String> values = _headers[name];
       bool fold = _foldHeader(name);
       var nameData = name.codeUnits;
       write(nameData);
-      write(const [_CharCode.COLON, _CharCode.SP]);
+      buffer[offset++] = _CharCode.COLON;
+      buffer[offset++] = _CharCode.SP;
       for (int i = 0; i < values.length; i++) {
         if (i > 0) {
           if (fold) {
-            write(const [_CharCode.COMMA, _CharCode.SP]);
+            buffer[offset++] = _CharCode.COMMA;
+            buffer[offset++] = _CharCode.SP;
           } else {
-            write(const [_CharCode.CR, _CharCode.LF]);
+            buffer[offset++] = _CharCode.CR;
+            buffer[offset++] = _CharCode.LF;
             write(nameData);
-            write(const [_CharCode.COLON, _CharCode.SP]);
+            buffer[offset++] = _CharCode.COLON;
+            buffer[offset++] = _CharCode.SP;
           }
         }
         write(values[i].codeUnits);
       }
-      write(const [_CharCode.CR, _CharCode.LF]);
-    });
+      buffer[offset++] = _CharCode.CR;
+      buffer[offset++] = _CharCode.LF;
+    }
     return offset;
   }