Add script attribute to Options class, providing the name of the running script.

corelib/src/options.dart

Index: corelib/src/options.dart
diff --git a/corelib/src/options.dart b/corelib/src/options.dart
index 7224f788cd16ed113e6f6611d47406bbe6058430..fea63d07fdb412395fcd8d8e99cdc1a32ed6225d 100644
--- a/corelib/src/options.dart
+++ b/corelib/src/options.dart
@@ -21,4 +21,9 @@ interface Options default RuntimeOptions {
    * this list.
    */
   List<String> get arguments();
+
+  /**
+   * Returns the path of the script being run in this isolate.
+   */
+  String get script();
 }