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

sdk/lib/_internal/dartdoc/test/dartdoc_test.dart

Index: sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
diff --git a/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart b/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
index be14aa095e94110befffb75cace2cbada11c822c..0bc58a39cf98fe1c02a9e80c114c89773e4a2fab 100644
--- a/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
+++ b/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
@@ -196,7 +196,7 @@ void _testRunDartDoc(List<String> libraryPaths, void eval(ProcessResult)) {
 
 /// The path to the root directory of the dartdoc entrypoint.
 String get _dartdocDir {
-  var dir = path.absolute(Platform.script);
+  var dir = path.absolute(Platform.script.toFilePath());
   while (path.basename(dir) != 'dartdoc') {
     if (!path.absolute(dir).contains('dartdoc') || dir == path.dirname(dir)) {
       fail('Unable to find root dartdoc directory.');