Try work around issues executing powershell scripts on windows on the bots

tools/testing/dart/browser_controller.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.
 library browser;
 
 import "dart:async";
 import "dart:convert" show UTF8, JSON;
 import "dart:core";
 import "dart:io";
 import "dart:math" show min;
@@ skipping 1889 matching lines @@
   final String date =
       new DateTime.now().toUtc().toIso8601String().replaceAll(':', '_');
   final screenshotName = 'ie_screenshot_${date}.png';
 
   // The "capture_screen.ps1" script is next to "test.dart" in "tools/"
   final powerShellScript =
       Platform.script.resolve('capture_screenshot.ps1').toFilePath();
   final screenshotFile =
       Platform.script.resolve('../$screenshotName').toFilePath();
 
-  final args = [powerShellScript, screenshotFile];
+  final args = [
+    '-ExecutionPolicy',
+    'ByPass',
+    '-File ',

[ricow1, 2017/04/12 10:25:42]

nit: remove  space after -File?

[kustermann, 2017/04/12 10:27:00]

Done.

+    powerShellScript,
+    screenshotFile
+  ];
   final ProcessResult result =
       await Process.run('powershell.exe', args, runInShell: true);
   if (result.exitCode != 0) {
     print('[$message] Failed to capture IE screenshot on windows: '
         'powershell.exe "${args.join(' ')}" returned with:\n'
         'exit code: ${result.exitCode}\n'
         'stdout: ${result.stdout}\n'
         'stderr: ${result.stderr}');
   } else {
     final storageUrl = 'gs://dart-temp-crash-archive/$screenshotName';
@@ skipping 10 matching lines @@
           'exit code: ${result.exitCode}\n'
           'stdout: ${result.stdout}\n'
           'stderr: ${result.stderr}');
     } else {
       print('[$message] Successfully uploaded screenshot to $storageUrl');
     }
     new File(screenshotFile).deleteSync();
   }
   print('--------------------------------------------------------------------');
 }