Implement completion in WithDirective.

pkg/analysis_server/test/services/completion/dart/optype_test.dart

Index: pkg/analysis_server/test/services/completion/dart/optype_test.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/optype_test.dart b/pkg/analysis_server/test/services/completion/dart/optype_test.dart
index 17cf6c41ab707ace27e56021e5caddf9b4d06679..0df0f1bbe53a63466a1a0838281c2519fbfb930e 100644
--- a/pkg/analysis_server/test/services/completion/dart/optype_test.dart
+++ b/pkg/analysis_server/test/services/completion/dart/optype_test.dart
@@ -1558,6 +1558,12 @@ class C2 {
     addTestSource('mth() { while (b^) {} }}');
     assertOpType(returnValue: true, typeNames: true);
   }
+
+  test_WithClause() {
+    // WithClause  ClassDeclaration
+    addTestSource('class x extends Object with ^\n{}');
+    assertOpType(typeNames: true);
+  }
 }
 
 class _TestSource implements Source {