Add tools/download_file.dart helper + download_contentshell shell/bat script included in the editor…

dart/tools/dartium/download_file.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.
+
+import 'dart:async';
+import 'dart:io';
+
+Future downloadFile(Uri url, String destination) {
+  var client = new HttpClient();
+  return client.getUrl(url)
+    .then((HttpClientRequest request) => request.close())
+    .then((HttpClientResponse response) {
+      if (response.statusCode != HttpStatus.OK) {
+        throw new Exception("Http status code (${response.statusCode}) "
+                            "was not 200. Aborting.");
+      }
+      var sink = new File(destination).openWrite();
+      return response.pipe(sink).then((_) {
+        client.close();
+    });
+  });
+}
+
+void main(List<String> arguments) {
+  die(String message) {
+    print(message);
+    exit(1);
+  }
+
+  if (arguments.length != 2) {
+    var scriptName = Platform.script.pathSegments.last;
+    die("Usage dart $scriptName <url> <destination-file>");
+  }
+
+  var url = Uri.parse(arguments[0]);
+  var destination = arguments[1];
+
+  if (!['http', 'https'].contains(url.scheme)) {
+    die("Unsupported scheme in uri $url");
+  }
+
+  print("Downloading $url to $destination.");
+  downloadFile(url, destination).then((_) {
+    print("Download finished.");
+  }).catchError((error) {
+    die("An unexpected error occured: $error.");
+  });
+}