Clicking on variable doesn't filter usage depending on assigning or using

## Steps to Reproduce
```
class Dummy {
bool variable;

void method() {
variable = true;
print("variable = $variable");
}
}
```

When you click on `variable` (by pressing cmd + mouse click), and you tap on filter option for assign values, it doesn't work, it still shows you the both results. Please see the screenshot for reference.

## Version info
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel beta, v1.0.1-pre.1, on Mac OS X 10.14 18A389, locale en-GB)
[✓] Android toolchain - develop for Android devices (Android SDK 28.0.3)
[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
[✓] Android Studio (version 3.2)
[✓] Connected device (1 available)

• No issues found!