Compute absolute script path at isolate startup.

tests/standalone/io/platform_test.dart

Index: tests/standalone/io/platform_test.dart
diff --git a/tests/standalone/io/platform_test.dart b/tests/standalone/io/platform_test.dart
index 85a7a4713ad3bb13dda7a64db72b8820ab799069..83645bb5f1f720952a3db136bf69614c8d5d2cd8 100644
--- a/tests/standalone/io/platform_test.dart
+++ b/tests/standalone/io/platform_test.dart
@@ -25,8 +25,14 @@ test() {
   var environment = Platform.environment;
   Expect.isTrue(environment is Map<String, String>);
   Expect.isTrue(Platform.executable.contains('dart'));
+  // Move directory to be sure script is correct.
+  var oldDir = Directory.current;
+  Directory.current = Directory.current.parent;
   Expect.isTrue(Platform.script.path.
                 endsWith('tests/standalone/io/platform_test.dart'));
+  Expect.isTrue(Platform.script.path.startsWith(oldDir.path));
+  // Restore dir.
+  Directory.current = oldDir;
   Directory packageRoot = new Directory(Platform.packageRoot);
   Expect.isTrue(packageRoot.existsSync());
   Expect.isTrue(new Directory("${packageRoot.path}/expect").existsSync());