fun String?.plus(other: Any?) : String

stdlib/src/jet/runtime/Intrinsics.java

@@ -0,0 +1,13 @@
+package jet.runtime;
+
+/**
+ * @author alex.tkachman
+ */
+public class Intrinsics {
+    private Intrinsics() {
+    }
+
+    public static String stringPlus(String self, Object other) {
+        return ((self == null) ? "null" : self) + ((other == null) ? "null" : other.toString());
+    }
+}