Repository: flutter
Issue: DateTime not working with localizationsDelegates
Labels: framework,f: material design,a: internationalization,dependency: dart,has reproducible steps,P2,found in release: 3.3,found in release: 3.6,team-design,triaged-design

## Steps to Reproduce

```
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:intl/intl.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: [
        const Locale('es', 'CO')
      ],
      locale: Locale('es', 'CO'),
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    DateTime day = DateTime(2020, 11, 7);
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              DateFormat('EEEE, d').format(day),
              style: Theme.of(context).textTheme.headline6,
            ),
          ],
        ),
      ),
    );
  }
}
```

**Expected results:** 
The text contains 'Sábado' (spanish for Saturday)

**Actual results:** 
Saturday


What I think that should be happening if that I set `locale: Locale('es', 'CO'),`, that will set the default locale for DateFormat too, but it is defaulting to `static String systemLocale = 'en_US';`.

I placed a debugger in the app and found this method:
```
  static String get defaultLocale {
    var zoneLocale = Zone.current[#Intl.locale] as String;
    return zoneLocale == null ? _defaultLocale : zoneLocale;
  }
```

I expect that this method will resolve to `locale` set in `MaterialApp()` constructor, but `zoneLocale` is `null` in this case

On the other hand if I run:
```
initializeDateFormatting();
Intl.defaultLocale = "es";
```

I get this error:
```
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following UnsupportedError was thrown building MediaQuery(MediaQueryData(size: Size(392.7, 759.3), devicePixelRatio: 2.8, textScaleFactor: 1.0, platformBrightness: Brightness.light, padding: EdgeInsets(0.0, 24.0, 0.0, 0.0), viewPadding: EdgeInsets(0.0, 24.0, 0.0, 0.0), viewInsets: EdgeInsets.zero, alwaysUse24HourFormat: false, accessibleNavigation: false, highContrast: false, disableAnimations: false, invertColors: false, boldText: false, navigationMode: traditional)):
Unsupported operation: Cannot set value in unmodifiable Map

The relevant error-causing widget was: 
  MaterialApp file:///Users/julian/dev/bug/lib/main.dart:16:12
When the exception was thrown, this was the stack: 
#0      _ImmutableMap.[]= (dart:core-patch/immutable_map.dart:74:5)
#1      initializeDateFormattingCustom (package:intl/date_symbol_data_custom.dart:35:19)
#2      loadDateIntlDataIfNotLoaded.<anonymous closure> (package:flutter_localizations/src/utils/date_localizations.dart:43:9)
#3      CastMap.forEach.<anonymous closure> (dart:_internal/cast.dart:288:8)
#4      _ImmutableMap.forEach (dart:core-patch/immutable_map.dart:43:8)
```

<details>
  <summary>Logs</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
      exception, please see if the error message includes enough information
      to explain how to solve the issue.
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```
[        ] Installing build/app/outputs/flutter-apk/app.apk... (completed in 0.9s)
[   +3 ms] executing: /Users/julian/Library/Android/sdk/platform-tools/adb -s emulator-5554 shell echo -n
b7976f0d88c427d8e71587aaa3245da38440ae2f > /data/local/tmp/sky.com.example.bug.sha1
[  +81 ms] sdk gphone x86 arm startApp
[   +7 ms] executing: /Users/julian/Library/Android/sdk/platform-tools/adb -s emulator-5554 shell am start -a
android.intent.action.RUN -f 0x20000000 --ez enable-background-compilation true --ez enable-dart-profiling true
--ez enable-checked-mode true --ez verify-entry-points true com.example.bug/com.example.bug.MainActivity
[ +123 ms] Starting: Intent { act=android.intent.action.RUN flg=0x20000000 cmp=com.example.bug/.MainActivity (has
extras) }
[   +1 ms] Waiting for observatory port to be available...
[+1517 ms] Observatory URL on device: http://127.0.0.1:35693/1JAfBsyqAx4=/
[   +1 ms] executing: /Users/julian/Library/Android/sdk/platform-tools/adb -s emulator-5554 forward tcp:0 tcp:35693
[   +9 ms] 63761
[        ] Forwarded host port 63761 to device port 35693 for Observatory
[   +5 ms] Caching compiled dill
[  +33 ms] Connecting to service protocol: http://127.0.0.1:63761/1JAfBsyqAx4=/
[   +1 ms] DDS is currently disabled due to https://github.com/flutter/flutter/issues/62507
[ +358 ms] Successfully connected to service protocol: http://127.0.0.1:63761/1JAfBsyqAx4=/
[        ] Waiting for sdk gphone x86 arm to report its views...
[   +7 ms] Waiting for sdk gphone x86 arm to report its views... (completed in 6ms)
[  +11 ms] DevFS: Creating new filesystem on the device (null)
[  +27 ms] DevFS: Created new filesystem on the device
(file:///data/user/0/com.example.bug/code_cache/bugQVYTJD/bug/)
[   +2 ms] Updating assets
[  +90 ms] Syncing files to device sdk gphone x86 arm...
[   +1 ms] Scanning asset files
[   +2 ms] <- reset
[        ] Compiling dart to kernel with 0 updated files
[        ] <- recompile package:bug/main.dart c23465ab-da43-4b5a-9298-52854cd7672b
[        ] <- c23465ab-da43-4b5a-9298-52854cd7672b
[  +66 ms] Updating files
[ +174 ms] DevFS: Sync finished
[        ] Syncing files to device sdk gphone x86 arm... (completed in 247ms)
[        ] Synced 1.6MB.
[   +1 ms] <- accept
[   +4 ms] Connected to _flutterView/0xee3c3680.
[   +1 ms] Flutter run key commands.
[   +2 ms] r Hot reload. 🔥🔥🔥
[   +1 ms] R Hot restart.
[        ] h Repeat this help message.
[        ] d Detach (terminate "flutter run" but leave application running).
[        ] c Clear the screen
[        ] q Quit (terminate the application on the device).
[        ] An Observatory debugger and profiler on sdk gphone x86 arm is available at:
http://127.0.0.1:63761/1JAfBsyqAx4=/

```

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
Analyzing bug...
No issues found! (ran in 5.0s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
flutter doctor -v                                                                                            ─╯
[✓] Flutter (Channel stable, 1.22.3, on Mac OS X 10.15.7 19H2, locale en-CO)
    • Flutter version 1.22.3 at /Users/julian/dev/flutter
    • Framework revision 8874f21e79 (9 days ago), 2020-10-29 14:14:35 -0700
    • Engine revision a1440ca392
    • Dart version 2.10.3

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/julian/Library/Android/sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 12.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.1, Build version 12A7403
    • CocoaPods version 1.10.0

[!] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)


[✓] Connected device (1 available)
    • sdk gphone x86 arm (mobile) • emulator-5554 • android-x86 • Android 11 (API 30) (emulator)

! Doctor found issues in 1 category.
```
(I have plugins installed, not sure what's going on)

</details>