Move exit from the builtin library to dart:io.

lib/compiler/implementation/lib/io.dart

Index: lib/compiler/implementation/lib/io.dart
diff --git a/lib/compiler/implementation/lib/io.dart b/lib/compiler/implementation/lib/io.dart
index a08372b8bb4354a41158c589a091c5171b426042..1355a6b31dfc591dff560005c1ffba1dd7a7b21d 100644
--- a/lib/compiler/implementation/lib/io.dart
+++ b/lib/compiler/implementation/lib/io.dart
@@ -115,6 +115,10 @@ class _Directory {
 class _DirectoryLister {
 }
 
+void _exit(int exitCode) {
+  throw new UnsupportedOperationException("exit($exitCode)");
+}
+
 class _Process {
   static Process start(String executable,
                        List<String> arguments,