Make root args to query(All)ByTestId nullable, add tests

lib/src/over_react_test/react_util.dart

@@ -422,6 +422,9 @@ Element? getComponentRootDomByTestId(dynamic root, String value, {String key = d
 /// Returns the [Element] of the first descendant of [root] that has its [key] html attribute value set to a
 /// space-delimited string containing [value].
 ///
+/// Throws if [root] or `findDomNode(root)` is `null`. [root] is kept nullable for convenience, since the input
+/// to this function is often a `dynamic` component instance value.
+///
 /// Setting [searchInShadowDom] to true will allow the query to search within ShadowRoots that use `mode:"open"`.
 /// [shadowDepth] will limit how many layers of ShadowDOM will searched. By default it will search infinitely deep, and this
 /// should only be needed if there are a lot of ShadowRoots within ShadowRoots.
@@ -450,7 +453,7 @@ Element? getComponentRootDomByTestId(dynamic root, String value, {String key = d
 ///     queryByTestId(renderedInstance, 'value'); // returns the `inner` `<div>`
 ///
 /// Related: [queryAllByTestId], [getComponentRootDomByTestId].
-Element? queryByTestId(Object root, String value, {String key = defaultTestIdKey, bool searchInShadowDom = false, int? shadowDepth}) {
+Element? queryByTestId(dynamic root, String value, {String key = defaultTestIdKey, bool searchInShadowDom = false, int? shadowDepth}) {
   ArgumentError.checkNotNull(root, 'root');
 
   final node = findDomNode(root);
@@ -464,6 +467,9 @@ Element? queryByTestId(Object root, String value, {String key = defaultTestIdKey
 
 /// Returns all descendant [Element]s of [root] that has their [key] html attribute value set to [value].
 ///
+/// Throws if [root] or `findDomNode(root)` is `null`. [root] is kept nullable for convenience, since the input
+/// to this function is often a `dynamic` component instance value.
+///
 /// Setting [searchInShadowDom] to true will allow the query to search within ShadowRoots that use `mode:"open"`.
 /// [shadowDepth] will limit how many layers of ShadowDOM will searched. By default it will search infinitely deep, and this
 /// should only be needed if there are a lot of ShadowRoots within ShadowRoots.
@@ -497,7 +503,7 @@ Element? queryByTestId(Object root, String value, {String key = defaultTestIdKey
 ///     </div>
 ///
 ///     queryAllByTestId(renderedInstance, 'value'); // returns both `inner` `<div>`s
-List<Element> queryAllByTestId(Object root, String value, {String key = defaultTestIdKey, bool searchInShadowDom = false, int? shadowDepth}) {
+List<Element> queryAllByTestId(dynamic root, String value, {String key = defaultTestIdKey, bool searchInShadowDom = false, int? shadowDepth}) {
   ArgumentError.checkNotNull(root, 'root');
 
   final node = findDomNode(root);

test/over_react_test/react_util_test.dart

@@ -809,6 +809,10 @@ main() {
         });
       });
 
+      test('throws when root is null', () {
+        expect(() => queryByTestId(null, 'unusedTestId'), throwsA(isA<ArgumentError>()));
+      });
+
       test('throws a helpful error when findDomNode(root) is null', () {
         var jacket = mount(RendersNothing()());
         expect(
@@ -934,6 +938,10 @@ main() {
         });
       });
 
+      test('throws when root is null', () {
+        expect(() => queryAllByTestId(null, 'unusedTestId'), throwsA(isA<ArgumentError>()));
+      });
+
       test('throws a helpful error when findDomNode(root) is null', () {
         var jacket = mount(RendersNothing()());
         expect(