Repository: flutter
Issue: Document how to use Intl in Flutter (specifying the locale obtained from the widget tree)
Labels: framework,a: internationalization,d: api docs,P2,team-framework,triaged-framework

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I have an app that today supports two languages:

- Swedish
- English

The language support is set up following [this guide](https://flutter.dev/docs/development/accessibility-and-localization/internationalization) 

The app has a LicensePage which previously had the same error as in #40353 and #28456 i.e. the LicencePage overrides the system locale. Not modifying the global `Intl.defaultLocale` solved this issue. Meaning implementing the changes recommended in [this PR](https://github.com/flutter/website/pull/3013/files)

Now I have another issue which causes the numbers and dates not to be formatted using the system locale (which it did before when i modified the the global `Intl.defaultLocale`). The rest of the app is translated correctly. And if I modify `Intl.defaultLocale` the dates and numbers are formatted correctly as well (until i open the LicensePage).

## Steps to Reproduce

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1. Create an app supporting two languages (Swedish and English) following [this guide](https://flutter.dev/docs/development/accessibility-and-localization/internationalization) using the intl package
2. Set the device language to Swedish
3. Call DateFormat i.e. 
```
import 'package:intl/intl.dart';

final date = DateTime.now()
DateFormat('dd MMM HH:mm').format(date.toLocal())
```
I would expect this to show the text in swedish in the local time zone.

## Logs

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<!-- Finally, paste the output of running `flutter doctor -v` here. -->
### flutter doctor -v
```
[✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Mac OS X 10.14.6 18G95, locale en-SE)
    • Flutter version 1.9.1+hotfix.2 at /Users/swedspot/Documents/development/flutter
    • Framework revision 2d2a1ffec9 (4 weeks ago), 2019-09-06 18:39:49 -0700
    • Engine revision b863200c37
    • Dart version 2.5.0

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/swedspot/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 10.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.3, Build version 10G8
    • CocoaPods version 1.7.5

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 39.0.3
    • Dart plugin version 191.8423
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] VS Code (version 1.38.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.5.1

[✓] Connected device (2 available)
    • Android SDK built for x86 • emulator-5554                        • android-x86 • Android 9 (API 28) (emulator)
    • iPhone X                  • EC967EA0-F0DC-4CA2-8C33-CF8CE6EE2207 • ios         • com.apple.CoreSimulator.SimRuntime.iOS-12-2 (simulator)
```