Make non-copying version of BytesBuidler and make file-reads faster.

runtime/bin/builtin.dart

 // Copyright (c) 2012, 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.
 
 library builtin;
 import 'dart:io';
 import 'dart:async';
 // import 'root_library'; happens here from C Code
 
 // The root library (aka the script) is imported into this library. The
@@ skipping 67 matching lines @@
   _httpRequestResponse = null;
   try {
     Uri requestUri = Uri.parse(uri);
     _client.getUrl(requestUri)
         .then((HttpClientRequest request) {
           request.persistentConnection = false;
           return request.close();
         })
         .then((HttpClientResponse response) {
           // Only create a ByteBuilder, if multiple chunks are received.
-          var bufferOrBuilder;
+          var builder = new BytesBuilder(copy: false);
           response.listen(
-            (data) {
-              if (bufferOrBuilder == null) {
-                bufferOrBuilder = data;
-              } else {
-                if (bufferOrBuilder is! BytesBuilder) {
-                  bufferOrBuilder = new BytesBuilder()
-                      ..add(bufferOrBuilder);
-                }
-                bufferOrBuilder.add(data);
-              }
-            },
+            builder.add,
             onDone: () {
-              var data = bufferOrBuilder;
-              if (data is BytesBuilder) data = data.takeBytes();
-              _requestCompleted(data, response);
+              _requestCompleted(builder.takeBytes(), response);
               // Close the client to stop any timers currently held alive.
               _client.close();
             },
             onError: _requestFailed);
         }).catchError((error) {
           _requestFailed(error);
         });
   } catch (error) {
     _requestFailed(error);
   }
@@ skipping 201 matching lines @@
   } else if (Platform.isWindows) {
     filename = '$name.dll';
   } else {
     _logResolution(
         'Native extensions not supported on ${Platform.operatingSystem}');
     throw 'Native extensions not supported on ${Platform.operatingSystem}';
   }
 
   return [path, filename, name];
 }