Add compile_from_dill test

tests/compiler/dart2js/output_collector.dart

Index: tests/compiler/dart2js/output_collector.dart
diff --git a/tests/compiler/dart2js/output_collector.dart b/tests/compiler/dart2js/output_collector.dart
index 0aa2480bda12603e4d3b365509ae6c8d0bd767a7..c81cb9993b7d073f7a8e6d5f1a65a6c6dfad6ed9 100644
--- a/tests/compiler/dart2js/output_collector.dart
+++ b/tests/compiler/dart2js/output_collector.dart
@@ -21,6 +21,10 @@ class BufferedOutputSink implements OutputSink {
     text = sb.toString();
     sb = null;
   }
+
+  String toString() {
+    return text ?? sb.toString();
+  }
 }
 
 class CloningOutputSink implements OutputSink {