Repository: flutter
Issue: `testwidgets()` in widget_tester.dart does not accept a string for skip parameter
Labels: a: tests,framework,c: proposal,P3,team-framework,triaged-framework

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## Steps to Reproduce
1. Write a test you want to skip
2. Try to skip the test, using a String for the reason for skipping
3. Error: The argument type 'String' can't be assigned to the parameter type 'bool.
![screen shot 2018-09-05 at 3 06 11 pm](https://user-images.githubusercontent.com/3046029/45123975-75017d80-b11d-11e8-990a-77ce417517aa.png)

From `widget_tester.dart`:
```
@isTest
void testWidgets(String description, WidgetTesterCallback callback, {
  bool skip = false,
  test_package.Timeout timeout
}) {
  final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized();
  ...
```

I believe the line: `bool skip = false,` 
should be: **`dynamic skip = false,`**

Making this change allows me to skip using a string.

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## Logs

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```
n/a
```

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```
No issues found! (ran in 2.7s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
jjerome:~ jmj$ flutter doctor -v
[✓] Flutter (Channel beta, v0.7.3, on Mac OS X 10.13.6 17G65, locale en-US)
    • Flutter version 0.7.3 at /Users/jmj/Documents/Development/flutter
    • Framework revision 3b309bda07 (8 days ago), 2018-08-28 12:39:24 -0700
    • Engine revision af42b6dc95
    • Dart version 2.1.0-dev.1.0.flutter-ccb16f7282

[✓] Android toolchain - develop for Android devices (Android SDK 27.0.3)
    • Android SDK at /Users/jmj/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 27.0.3
    • ANDROID_HOME = /Users/jmj/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_152-release-1024-b01)
    • All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 9.4.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 9.4.1, Build version 9F2000
    • ios-deploy 1.9.2
    • CocoaPods version 1.5.3

[✓] Android Studio (version 3.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 28.0.1
    • Dart plugin version 173.4700
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)

[!] IntelliJ IDEA Community Edition (version 2018.1.3)
    • IntelliJ at /Applications/IntelliJ IDEA CE.app
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • For information about installing plugins, see
      https://flutter.io/intellij-setup/#installing-the-plugins

[✓] Connected devices (1 available)
    • Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

! Doctor found issues in 1 category.
```