Add dart-json snapshot test

__tests__/__snapshots__/dart-json.test.ts.snap

@@ -0,0 +1,9 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`dart-json tests matches report snapshot 1`] = `
+Object {
+  "annotations": Array [],
+  "summary": "**6** tests were completed in **3.760s** with **1** passed, **1** skipped and **4** failed.",
+  "title": "Dart tests ❌",
+}
+`;