Coverage doesn't parse correctly if the lcov file contains an absolute file path on Windows

While testing enabling coverage for Dart, I noticed that the lcov files can sometimes have absolute file paths:

```
SF:D:\Dev\Test Projects\coverage_testing\dart_package_1\lib\dart_package_1.dart
```

And because we split on `:` and then check for 2 fields, this fails. We need to only split on the first `:`.