Change dart:io Platform.script to return a URI. Change all uses of Platform.script to work with th…

sdk/lib/_internal/compiler/implementation/dart2js.dart

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2js.dart b/sdk/lib/_internal/compiler/implementation/dart2js.dart
index a827bf6ea03e02ee597e117055cf8118d5adc0b5..e4d6cafe5a00064209fa65c55e435301d7cceb5e 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -454,8 +454,8 @@ void fail(String message) {
 
 Future compilerMain(List<String> arguments) {
   var root = uriPathToNative("/$LIBRARY_ROOT");
-  arguments =
-      <String>['--library-root=${Platform.script}$root']..addAll(arguments);
+  arguments = <String>['--library-root=${Platform.script.toFilePath()}$root']
+      ..addAll(arguments);
   return compile(arguments);
 }