Hightlight not work on flutter sdk code

The project code is highlighted, the sdk source code is not highlighted.

> project code

> flutter sdk source code

## Version info

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel unknown, v1.0.0, on Mac OS X 10.14.3 18D109, locale zh-Hans-CN)
[!] Android toolchain - develop for Android devices (Android SDK 28.0.3)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses
[!] iOS toolchain - develop for iOS devices (Xcode 10.1)
! CocoaPods out of date (1.5.0 is recommended).
CocoaPods is used to retrieve the iOS platform side's plugin code that responds to your plugin usage on the Dart
side.
Without resolving iOS dependencies with CocoaPods, plugins will not work on iOS.
For more info, see https://flutter.io/platform-plugins
To upgrade:
brew upgrade cocoapods
pod setup
[✓] Android Studio (version 3.3)
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] IntelliJ IDEA Ultimate Edition (version 2018.3.5)
[✓] VS Code (version 1.31.1)
[✓] Connected device (1 available)

! Doctor found issues in 2 categories.
```