Frog has hardwired access to NullPointerException: default argument can only be null.

corelib/src/exceptions.dart

Index: corelib/src/exceptions.dart
===================================================================
--- corelib/src/exceptions.dart	(revision 6435)
+++ corelib/src/exceptions.dart	(working copy)
@@ -121,13 +121,14 @@
 
 
 class NullPointerException implements Exception {
-  const NullPointerException([this.functionName,
-                              this.arguments = const[]]);
+  const NullPointerException([this.functionName, this.arguments]);
   String toString() {
     StringBuffer sb = new StringBuffer();
-    for (var arg in arguments) {
-      if (!sb.isEmpty()) sb.add(", ");
-      sb.add(arg);
+    if (arguments != null) {
+      for (var arg in arguments) {
+        if (!sb.isEmpty()) sb.add(", ");
+        sb.add(arg);
+      }
     }
     if (functionName == null) {
       return exceptionName;