better error message when Navigator is used with no NavigatorState in the context

packages/flutter/lib/src/widgets/navigator.dart

@@ -157,6 +157,11 @@ class Navigator extends StatefulComponent {
 
   static void openTransaction(BuildContext context, NavigatorTransactionCallback callback) {
     NavigatorState navigator = context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
+    assert(() {
+      if (navigator == null)
+        throw new WidgetError('openTransaction called with a context that does not include a Navigator. The context passed to the Navigator.openTransaction() method must be that of a widget that is a descendant of a Navigator widget.');
+      return true;
+    });
     navigator.openTransaction(callback);
   }
 

packages/flutter/test/widget/navigator_test.dart

@@ -40,6 +40,29 @@ class SecondComponentState extends State<SecondComponent> {
   }
 }
 
+typedef void ExceptionCallback(exception);
+
+class ThirdComponent extends StatelessComponent {
+  ThirdComponent({ this.targetKey, this.onException });
+
+  final Key targetKey;
+  final ExceptionCallback onException;
+
+  Widget build(BuildContext context) {
+    return new GestureDetector(
+      key: targetKey,
+      onTap: () {
+        try {
+          Navigator.openTransaction(context, (_) { });
+        } catch (e) {
+          onException(e);
+        }
+      },
+      behavior: HitTestBehavior.opaque
+    );
+  }
+}
+
 void main() {
   test('Can navigator navigate to and from a stateful component', () {
     testWidgets((WidgetTester tester) {
@@ -73,4 +96,21 @@ void main() {
       expect(tester.findText('Y'), isNull);
     });
   });
+
+  test('Navigator.openTransaction fails gracefully when not found in context', () {
+    testWidgets((WidgetTester tester) {
+      Key targetKey = new Key('foo');
+      dynamic exception;
+      Widget widget = new ThirdComponent(
+        targetKey: targetKey,
+        onException: (e) {
+          exception = e;
+        }
+      );
+      tester.pumpWidget(widget);
+      tester.tap(tester.findElementByKey(targetKey));
+      expect(exception, new isInstanceOf<WidgetError>());
+      expect('$exception', startsWith('openTransaction called with a context'));
+    });
+  });
 }