This addresses the problem in https://github.com/flutter/flutter/issues/12847 which changed slightly over time.
Today, `Platform.script` does not give an empty `file` URI, it gives something like `file://path/to/package/main.dart` _regardless of how the file is actually named_.
After this change, it will give the absolute path to the file being run under test.
So before this change, the new test would have a URI like
```
file:///Users/dnfield/src/flutter/engine/src/main.dart
```
And now it has
```
file:///Users/dnfield/src/flutter/engine/src/out/host_debug_unopt_arm64/gen/platform_test.dart.dill
```
This is going to be helpful in generating relative paths from the test file.
670b5bd8e7