Avoid spurious "The compiler crashed" messages in batch mode.

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 ca0ef6af424ad412305847b7eae9ad55c5d2990d..a4858f6dfe6ff66d3eb4aacc093ac5dfbea084a5 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -654,6 +654,9 @@ var compileFunc = api.compile;
 
 Future<api.CompilationResult> internalMain(List<String> arguments) {
   Future onError(exception, trace) {
+    // If we are already trying to exit, just continue exiting.
+    if (exception == _EXIT_SIGNAL) throw exception;
+
     try {
       print('The compiler crashed: $exception');
     } catch (ignored) {