Move exit from the builtin library to dart:io.

lib/compiler/implementation/lib/mock.dart

Index: lib/compiler/implementation/lib/mock.dart
diff --git a/lib/compiler/implementation/lib/mock.dart b/lib/compiler/implementation/lib/mock.dart
index d5e601c6756d9f6fa88b4a63743e29dcbbf5f3ba..894846c12f106e9c0f014070c4390a1430f6beaf 100644
--- a/lib/compiler/implementation/lib/mock.dart
+++ b/lib/compiler/implementation/lib/mock.dart
@@ -59,11 +59,6 @@ class _InternalByteArray {
   }
 }
 
-// TODO(ahe): This definitely does not belong in the core library.
-void exit(int exitCode) {
-  throw new UnsupportedOperationException("exit($exitCode)");
-}
-
 class _Expando<T> implements Expando<T> {
 
   final String name;