[Time Conductor] Fix date picker toggle

nasa/openmctweb#297; avoid flag toggled to display date-time
picker becoming lost in a child scope.

platform/commonUI/general/test/controllers/DateTimeFieldControllerSpec.js

@@ -91,6 +91,10 @@ define(
                 expect(mockScope.textValue).toEqual("1977-05-25 17:30:00");
             });
 
+            it("exposes toggle state for date-time picker", function () {
+                expect(mockScope.picker.active).toBe(false);
+            });
+
             describe("when user input is invalid", function () {
                 var newText, oldValue;