Add better error reporting to utils tests.

tests/utils/source_mirrors_test.dart

Index: tests/utils/source_mirrors_test.dart
diff --git a/tests/utils/source_mirrors_test.dart b/tests/utils/source_mirrors_test.dart
index c61fafb116ed5dccc9a35a2a1ab0a20a414e8128..a29da99bf9f3d3c09d8e02ce75f81dfa52396238 100644
--- a/tests/utils/source_mirrors_test.dart
+++ b/tests/utils/source_mirrors_test.dart
@@ -30,7 +30,7 @@ main() {
         print(' $name:$declaration');
       });
     });
-  }, onError: (e) {
-      throw 'Analysis failed';
+  }, onError: (e, s) {
+      throw 'Analysis failed: $e\n$s';
   }).then(asyncSuccess);
 }