fix(chonge-detection): don't call reactionFn on deleted scope

Author: mhevery

lib/change_detection/watch_group.dart

@@ -413,7 +413,6 @@ class RootWatchGroup extends WatchGroup {
     Watch dirtyWatch = _dirtyWatchHead;
     _dirtyWatchHead = null;
     RootWatchGroup root = _rootGroup;
-    root._removeCount = 0;
     try {
       while(dirtyWatch != null) {
         count++;
@@ -430,6 +429,7 @@ class RootWatchGroup extends WatchGroup {
       }
     } finally {
       _dirtyWatchTail = null;
+      root._removeCount = 0;
     }
     if (processStopwatch != null) processStopwatch..stop()..increment(count);
     return count;

test/core/scope_spec.dart

@@ -740,6 +740,18 @@ void main() {
         rootScope.apply();
         expect(log).toEqual(['root destroy', 'root2 destroy']);
       }));
+
+
+      it('should not call reaction fn when destroyed', inject((RootScope scope) {
+        var testScope = scope.createChild({});
+        bool called = false;
+        testScope.watch('items', (_, __) {
+          called = true;
+        });
+        testScope.destroy();
+        scope.apply();
+        expect(called).toBeFalsy();
+      }));
     });