Make dartdoc compile.

dart/frog/leg/lib/mock.dart

Index: dart/frog/leg/lib/mock.dart
diff --git a/dart/frog/leg/lib/mock.dart b/dart/frog/leg/lib/mock.dart
index 74421d3b245313f6212ade9507870856e97d41ad..f61ef40932533ecc072cfd38b67577037258d7a1 100644
--- a/dart/frog/leg/lib/mock.dart
+++ b/dart/frog/leg/lib/mock.dart
@@ -26,6 +26,7 @@ class StaticResolutionException implements Exception {}
 
 void assert(condition) {}
 
+// TODO(ahe): Not sure ByteArray belongs in the core library.
 interface ByteArray extends List default _InternalByteArray {
   ByteArray(int length);
 }
@@ -35,3 +36,8 @@ class _InternalByteArray {
     throw new UnsupportedOperationException("new ByteArray($length)");
   }
 }
+
+// TODO(ahe): This definitely does not belong in the core library.
+void exit(int exitCode) {
+  throw new UnsupportedOperationException("exit($exitCode)");
+}