Change dart:io Platform.script to return a Uri.

samples/sample_extension/test/sample_extension_test.dart

Index: samples/sample_extension/test/sample_extension_test.dart
diff --git a/samples/sample_extension/test/sample_extension_test.dart b/samples/sample_extension/test/sample_extension_test.dart
index aa3c620407e288e8256896054ee43711045082e7..9c0a42d30468c79d2a4caafba5165a0ad5df1a88 100644
--- a/samples/sample_extension/test/sample_extension_test.dart
+++ b/samples/sample_extension/test/sample_extension_test.dart
@@ -39,12 +39,11 @@ String getNativeLibraryPath(String buildDirectory) {
 }
 
 void main() {
-  String scriptDirectory = dirname(Platform.script);
   String buildDirectory = dirname(Platform.executable);
   Directory tempDirectory =
       Directory.systemTemp.createTempSync('sample_extension_');
   String testDirectory = tempDirectory.path;
-  String sourceDirectory = join(scriptDirectory, '..');
+  String sourceDirectory = Platform.script.resolve('..').toFilePath();
 
   // Copy sample_extension shared library, sample_extension dart files and
   // sample_extension tests to the temporary test directory.