Return empty contents for not existing sources.

pkg/analyzer/test/src/task/general_test.dart

Index: pkg/analyzer/test/src/task/general_test.dart
diff --git a/pkg/analyzer/test/src/task/general_test.dart b/pkg/analyzer/test/src/task/general_test.dart
index a043c84242bc7872ec0911a4c7277869ada7dd8b..e7b6e92d72dad9cb4b84921cd2f0bc99870d56c0 100644
--- a/pkg/analyzer/test/src/task/general_test.dart
+++ b/pkg/analyzer/test/src/task/general_test.dart
@@ -73,8 +73,10 @@ class GetContentTaskTest extends EngineTestCase {
     GetContentTask task = new GetContentTask(context, target);
     when(context.getContents(target)).thenThrow('My exception!');
     task.perform();
-    expect(task.caughtException, isNotNull);
-    expect(task.outputs, isEmpty);
+    expect(task.caughtException, isNull);
+    expect(task.outputs, hasLength(2));
+    expect(task.outputs[CONTENT], '');
+    expect(task.outputs[MODIFICATION_TIME], -1);
   }
 }