Introduce LegacyApp to simplify porting run tests. scala#559

Author: DarkDimius

src/dotty/runtime/LegacyApp.scala

@@ -0,0 +1,9 @@
+package dotty.runtime
+
+
+/**
+ * replaces the `scala.App` class which relies on `DelayedInit` functionality, not supported by Dotty.
+ */
+class LegacyApp {
+  def main(args: Array[String]): Unit = ()
+}