Some cleanup of the Sky analyzer script

* If no source path is provided, then run the analyzer on the Sky unit tests
* Fix the filter for errors found in pub cache packages
* Generalize the filter for the analyzer's "xx errors/warnings/hints found" status message

Also fix a test that caused a warning in the analyzer.

packages/unit/.analysis_options

-analyzer:
-  exclude:
-    - 'test/engine/append_child_test.dart'

packages/unit/test/engine/append_child_test.dart

@@ -10,7 +10,7 @@ void main() {
   test("should throw with invalid arguments", () {
     var parent = document.createElement("div");
     expect(() {
-      parent.appendChild();
+      Function.apply(parent.appendChild, []);
     }, throws);
     expect(() {
       parent.appendChild(null);