Repository: flutter
Issue: Windows: Unable to paste in a focused TextField.
Labels: a: text input,framework,platform-windows,a: desktop,P2,team-windows,triaged-windows

## Steps to Reproduce
Code:
```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'test',
      theme: ThemeData.dark(),
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final focusNode = FocusNode();
  final txtController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: TextField(
          focusNode: focusNode,
          autofocus: true,
          controller: txtController,
          onSubmitted: (token) => setState(() {
            txtController.clear();
            focusNode.requestFocus();
          }),
        ),
      ),
    );
  }
}

```
1. copy some text such as "aaa".
2. run this code
3. type some text into the textfield.
4. press Enter and ctrl+v

**Expected results:** <!-- what did you want to see? -->

the copied text should be pasted into the textfield.

if you click the textfield and then ctrl+v, it will be successfull.

**Actual results:** <!-- what did you see? -->
paste failed with error in console:
```
[ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: RangeError: Value not in range: -1
#0      _StringBase.substring (dart:core-patch/string_patch.dart:393:7)
#1      TextRange.textBefore (dart:ui/text.dart:1533:17)
#2      RenderEditable._handleShortcuts (package:flutter/src/rendering/editable.dart:782:33)
<asynchronous suspension>
#3      RenderEditable._handleKeyEvent (package:flutter/src/rendering/editable.dart:545:7)
#4      RawKeyboard._handleKeyEvent (package:flutter/src/services/raw_keyboard.dart:587:17)
#5      BasicMessageChannel.setMessageHandler.<anonymous closure> (package:flutter/src/services/platform_channel.dart:75:49)
#6      _DefaultBinaryMessenger.handlePlatformMessage (package:flutter/src/services/binding.dart:283:33)
#7      _invoke3.<anonymous closure> (dart:ui/hooks.dart:280:15)
#8      _rootRun (dart:async/zone.dart:1190:13)
#9      _CustomZone.run (dart:async/zone.dart:1093:19)
#10     _CustomZone.runGuarded (dart:async/zone.dart:997:7)
#11     _invoke3 (dart:ui/hooks.dart:279:10)
#12     _dispatchPlatformMessage (dart:ui/hooks.dart:154:5)
```
<details>
  <summary>Logs</summary>

`flutter doctor -v`

```
[√] Flutter (Channel master, 1.21.0-8.0.pre.98, on Microsoft Windows [Version 10.0.17763.1039], locale en-US)
    • Flutter version 1.21.0-8.0.pre.98 at D:\Software\flutter
    • Framework revision 77b4505c80 (19 hours ago), 2020-08-06 18:51:02 -0700
    • Engine revision cd3ea1e839
    • Dart version 2.10.0 (build 2.10.0-3.0.dev fcafd43f2c)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at D:\Software\AndroidSDK
    • Platform android-29, build-tools 29.0.2
    • Java binary at: D:\Software\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[X] Chrome - develop for the web (Cannot find Chrome executable at .\Google\Chrome\Application\chrome.exe)
    ! Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.

[√] Visual Studio - develop for Windows (Visual Studio Enterprise 2019 16.6.5)
    • Visual Studio at D:\Software\VS2019
    • Visual Studio Enterprise 2019 version 16.6.30320.27
    • Windows 10 SDK version 10.0.19041.0

[√] Android Studio (version 4.0)
    • Android Studio at D:\Software\Android Studio
    • Flutter plugin version 47.1.2
    • Dart plugin version 193.7361
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] Connected device (2 available)
    • Windows (desktop) • windows    • windows-x64    • Microsoft Windows [Version 10.0.17763.1039]
    • Web Server (web)  • web-server • web-javascript • Flutter Tools
```

</details>