Unable to see test icons in gutter

Unable to see any gutter icons for test files.

## Steps to Reproduce

Open `*_test.dart` files and no Run icons show up in the gutter.


Notice that on line no. **32**, there are no icons for running the test.

## Version info

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, v1.2.1, on Mac OS X 10.14.4 18E226, locale en-GB)
[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] iOS toolchain - develop for iOS devices (Xcode 10.2.1)
[!] Android Studio (version 3.4)
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] IntelliJ IDEA Ultimate Edition (version 2019.1.1)
[✓] VS Code (version 1.33.1)
[✓] Connected device (1 available)

! Doctor found issues in 1 category.

Reproduce the problem by opening a *_test.dart file in IntelliJ IDEA and checking the gutter around line 32. Start by tracing the Flutter plugin's test-gutter integration and compare the behavior with the reported Flutter and IntelliJ versions. Done means test run icons appear for applicable test files and launch successfully.

Written by the indexing model from the issue text.