Repository: flutter
Issue: showDatePicker has date format issues when DatePickerEntryMode.input is selected
Labels: a: text input,framework,f: material design,f: date/time picker,customer: crowd,P2,team-design,triaged-design

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## Steps to Reproduce

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![20200728_141801](https://user-images.githubusercontent.com/3411015/88660233-0649b680-d0df-11ea-8525-1671743e8fcf.gif)


1. Call showDatePicker method and choose edit mode in dialog. (Tested with 'tr' locale)
2. Try to change the TextField manually. It changes from dot(.) format to slashes(/). Then when you enter, you get invalid format error. If you delete them all, then you can only enter digits because you don't have control over keyboard.



**Expected results:** <!-- what did you want to see? -->
What I expect from this feature(DatePickerEntryMode.input) is to be able to disable/enable it. If I can't disable it, I want to control it. I want to be able to specify date format, control keyboard etc.

**Actual results:** <!-- what did you see? -->
As specified at step 2, when you try to change the TextField manually, It changes from dot(.) format to slashes(/). Then when you enter, you get invalid format error. If you delete them all, then you can only enter digits because you don't have control over keyboard.

<details>
  <summary>Logs</summary>

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```
[✓] Flutter (Channel stable, v1.17.5, on Mac OS X 10.15.5 19F101, locale en-TR)
    • Flutter version 1.17.5 at /Users/figengungor/Documents/development/flutter
    • Framework revision 8af6b2f038 (4 weeks ago), 2020-06-30 12:53:55 -0700
    • Engine revision ee76268252
    • Dart version 2.8.4

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/figengungor/Library/Android/sdk
    • 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_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.5)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.5, Build version 11E608c
    • CocoaPods version 1.8.4

[✓] Android Studio (version 4.0)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 47.1.2
    • Dart plugin version 193.7361
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[!] Connected device
    ! No devices available

! Doctor found issues in 1 category.
```

</details>