[dash] Fix broken external links

[chalin]

In particular note the 400 status codes.

Related: #1412

http://localhost:4002/development/ui/widgets/widgetindex
- (35:2) <link> => https://flutter-io-staging-1.firebaseapp.com/development/ui/widgets/widget (HTTP 404)

http://localhost:4002/get-started/flutter-for/android-devs
- (1355:0) 'StackOve..' => https://stackoverflow.com/questions/46241071/create-signature-area-%0Afor-mobile-app-in-dart-flutter (HTTP 400)

http://localhost:4002/get-started/flutter-for/ios-devs
- (1337:0) 'StackOve..' => https://stackoverflow.com/questions/46241071/create-signature-area-%0Afor-mobile-app-in-dart-flutter (HTTP 400)
- (2160:0) 'AppLifec..' => https://docs.flutter.io/flutter/dart-ui%0A/AppLifecycleState-class.html (connection failed)

http://localhost:4002/get-started/flutter-for/react-native-devs
- (2120:0) 'TextStyle' => https://docs.flutter.io/flutter/dart-ui/TextStyle-class.html (connection failed)
- (2282:0) 'BuildCon..' => https://docs.flutter.io/flutter/widgets/BuildContext-class.html (connection failed)
- (2775:0) 'context' => https://docs.flutter.io/flutter/widgets/BuildContext-class.html (connection failed)

http://localhost:4002/get-started/flutter-for/web-devs
- (1015:0) 'BoxConst..' => https://docs.flutter.io/flutter/rendering/BoxConstraints-class.html (connection failed)
- (1391:0) 'BorderRa..' => https://docs.flutter.io/flutter/painting/BorderRadius-class.html (connection failed)
- (1453:0) 'BoxShadow' => https://docs.flutter.io/flutter/painting/BoxShadow-class.html (connection failed)

http://localhost:4002/get-started/flutter-for/xamarin-forms-devs
- (1519:0) 'StackOve..' => https://stackoverflow.com/questions/46241071/create-signature-area-%0Afor-mobile-app-in-dart-flutter (HTTP 400)

http://localhost:4002/get-started/install/windows
- (862:0) 'PowerShe..' => https://docs.microsoft.com/en-us/powershell/scripting/_installing-windows-powershell#upgrading-existing-windows-powershell (HTTP 404)
- (862:0) 'PowerShe..' => https://docs.microsoft.com/en-us/powershell/scripting/_installing-windows-powershell#upgrading-existing-windows-powershell (HTTP 404)

@filiph, also notice the connection failures, these may be related to dart-lang/site-www#1107.

cc @Sfshaza

[Sfshaza]

Is this still true? I haven't seen it.

[chalin]

It probably still is true. We don't have external link checking enabled for dash yet.

[sfshaza2]

Changing to P0.

[chalin]

All fixed!