fix($q): allow third-party promise libraries

For testing, it can be useful to overwrite `$q` with other promise
implementions, such as Bluebird + angular-bluebird-promises. This broke
in v1.6.x with "TypeError: Cannot set property 'pur' of undefined".

Closes #16164

Closes #16471

src/ng/q.js

@@ -683,5 +683,11 @@ function markQStateExceptionHandled(state) {
   state.pur = true;
 }
 function markQExceptionHandled(q) {
-  markQStateExceptionHandled(q.$$state);
+  // Built-in `$q` promises will always have a `$$state` property. This check is to allow
+  // overwriting `$q` with a different promise library (e.g. Bluebird + angular-bluebird-promises).
+  // (Currently, this is the only method that might be called with a promise, even if it is not
+  // created by the built-in `$q`.)
+  if (q.$$state) {
+    markQStateExceptionHandled(q.$$state);
+  }
 }