Repository: flutter
Issue: showDatePicker ignore ThemeData.primaryColor
Labels: framework,f: material design,f: date/time picker,a: annoyance,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-design,triaged-design

## Steps to Reproduce

1. Update flutter from 1.12 to 1.17 channel stable
2. Our widget with showDatePicker does no more work as before

Code :

```
Future<DateTime> _getDate(BuildContext context, [DateTime firstDate, DateTime lastDate, DateTime initialDate]) async{
            DateTime fDate = firstDate != null ? DateTime(firstDate.year, firstDate.month, firstDate.day) : DateTime.now().subtract(Duration(days: 36500));
            DateTime lDate = lastDate != null ? DateTime(lastDate.year, lastDate.month, lastDate.day) : DateTime.now().add(Duration(days: 3650));
            DateTime iDate = initialDate != null ? DateTime(initialDate.year, initialDate.month, initialDate.day) : null;

            return await showDatePicker(
                initialDatePickerMode: isDateYear ? DatePickerMode.year : DatePickerMode.day,
                context: context,
                firstDate: fDate,
                lastDate: lDate,
                initialDate: (iDate != null && (iDate.isAfter(fDate) || iDate == fDate))
                             ? iDate
                             : (iDate == null && DateTime.now().isBefore(fDate))
                                    ? fDate
                                    : DateTime.now(),
                locale: Locale('fr', 'FR'),
                **//code added for 1.17**
                builder: (BuildContext context, Widget child) {
                    return Theme(
                        data: Theme.of(context),
                        child: child,
                        isMaterialAppTheme: true,
                    );
                },
            );
        }
```
There are no errors but

- theme is lost
- "SELECT DATE" displayed in the header, it's in english whereas my locale is french. And we can't set it to null to remove that helpText.

**Expected results:** <!-- what did you want to see? -->
![Screenshot_20200602-161416](https://user-images.githubusercontent.com/38036065/83531983-f0a87e00-a4ed-11ea-9731-8ba697bc753f.jpg)


**Actual results:** <!-- what did you see? -->
![image](https://user-images.githubusercontent.com/38036065/83532028-0158f400-a4ee-11ea-9da4-0e14234f5a5e.png)


<details>

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[✓] Flutter (Channel stable, v1.17.2, on Mac OS X 10.14.6 18G4032, locale fr-FR)
    • Flutter version 1.17.2 at /Users/s28091/Tools/flutter
    • Framework revision 5f21edf8b6 (5 days ago), 2020-05-28 12:44:12 -0700
    • Engine revision b851c71829
    • Dart version 2.8.3

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at /Users/s28091/Library/Android/sdk
    • Platform android-R, build-tools 29.0.3
    • ANDROID_HOME = /Users/s28091/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.3.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.3.1, Build version 11C505
    • CocoaPods version 1.9.2

[✓] Android Studio (version 3.6)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 45.1.1
    • Dart plugin version 192.8052
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)

[✓] IntelliJ IDEA Ultimate Edition (version 2020.1.1)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    • Flutter plugin version 45.1.3
    • Dart plugin version 201.7223.99

[✓] Proxy Configuration
    • HTTP_PROXY is set
    • NO_PROXY is localhost,127.0.0.1
    • NO_PROXY contains 127.0.0.1
    • NO_PROXY contains localhost

[✓] Connected device (2 available)
    • Android SDK built for x86 • emulator-5554                        • android-x86 • Android 10 (API 29)
      (emulator)
    • iPhone 11 Pro Max         • F27C72A5-9D88-40D0-8F6B-5B624E0239A6 • ios         •
      com.apple.CoreSimulator.SimRuntime.iOS-13-3 (simulator)

• No issues found!
```

</details>

Thanks