Ensure isolates have a root library so createMirrorSystem() and spawnUri() are happy.

sky/tests/dart/mirror-system.sky

Index: sky/tests/dart/mirror-system.sky
diff --git a/sky/tests/dart/mirror-system.sky b/sky/tests/dart/mirror-system.sky
new file mode 100644
index 0000000000000000000000000000000000000000..7d751e516dca32b0e303735a73d66d78624ed697
--- /dev/null
+++ b/sky/tests/dart/mirror-system.sky
@@ -0,0 +1,14 @@
+<script>
+import "dart:mirrors";
+import "dart:sky.internals" as internals;
+
+// Regression test for the embedder not setting up the root library.
+// Used to crash.
+
+void main() {
+  if (currentMirrorSystem().isolate.rootLibrary == null) {
+    throw "Missing root library";
+  }
+  internals.notifyTestComplete("PASS");
+}
+</script>>