Add the ability to get the current script or project path when using the new test runner

http://stackoverflow.com/questions/30214563/get-current-script-path-or-current-project-path-using-new-test-runner
I am porting old vm unittest files using the new test package. Some relies on input files in sub directories of my test folder. Before I was using Platform.script to find the location of such files. This works fine when using

```
$ dart test/my_test.dart
```

However using

```
$ pub run test
```

this is now pointing to a temp folder (tmp/dart_test_xxxx/runInIsolate.dart). I am unable to locate my test input files anymore. I cannot rely on the current path as I might run the test from a different working directory.

Is there a way to find the location of my_test.dart (or event the project root path), from which I could derive the locations of my files?

Start by reading the new test runner entry point invoked by `pub run test` and compare its behavior with `dart test/my_test.dart`. Investigate how `Platform.script` becomes `tmp/dart_test_xxxx/runInIsolate.dart`, using `my_test.dart` and the referenced Stack Overflow question as context. Done means tests can reliably locate files relative to the test script or project without depending on the working directory.

Written by the indexing model from the issue text.