dart2js: Use unique instance as exit signal.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 4d5edf8db3aba0c39610f65adb07a2a00b63c77b..01cb3bd6cd548fa0005fc41c2582cbab03d8f247 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -673,7 +673,11 @@ Future<api.CompilationResult> internalMain(List<String> arguments) {
   }
 }
 
-const _EXIT_SIGNAL = const Object();
+class _ExitSignal {
+  const _ExitSignal();
+}
+
+const _EXIT_SIGNAL = const _ExitSignal();
 
 void batchMain(List<String> batchArguments) {
   int exitCode;