add an assert to validate the RefreshIndicator.onRefresh result (#13198)

* add an assert to validate the RefreshIndicator.onRefresh result

* add a test for RefreshIndicator.onRefresh assert

* switch to using FlutterError.reportError, FlutterErrorDetails, and FlutterError

packages/flutter/lib/src/material/refresh_indicator.dart

@@ -326,7 +326,22 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
             _mode = _RefreshIndicatorMode.refresh;
           });
 
-          widget.onRefresh().whenComplete(() {
+          final Future<Null> refreshResult = widget.onRefresh();
+          assert(() {
+            if (refreshResult == null)
+              FlutterError.reportError(new FlutterErrorDetails(
+                exception: new FlutterError(
+                  'The onRefresh callback returned null.\n'
+                  'The RefreshIndicator onRefresh callback must return a Future.'
+                ),
+                context: 'when calling onRefresh',
+                library: 'material library',
+              ));
+            return true;
+          });
+          if (refreshResult == null)
+            return;
+          refreshResult.whenComplete(() {
             if (mounted && _mode == _RefreshIndicatorMode.refresh) {
               completer.complete();
               _dismiss(_RefreshIndicatorMode.done);

packages/flutter/test/material/refresh_indicator_test.dart

@@ -349,4 +349,33 @@ void main() {
     expect(completed1, true);
     expect(completed2, true);
   });
+
+  testWidgets('RefreshIndicator - onRefresh asserts', (WidgetTester tester) async {
+    refreshCalled = false;
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new RefreshIndicator(
+          onRefresh: () {
+            refreshCalled = true;
+            // Missing a returned Future value here.
+          },
+          child: new ListView(
+            physics: const AlwaysScrollableScrollPhysics(),
+            children: <String>['A', 'B', 'C', 'D', 'E', 'F'].map((String item) {
+              return new SizedBox(
+                height: 200.0,
+                child: new Text(item),
+              );
+            }).toList(),
+          ),
+        ),
+      ),
+    );
+
+    await tester.fling(find.text('A'), const Offset(0.0, 300.0), 1000.0);
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1)); // finish the scroll animation
+    expect(refreshCalled, true);
+    expect(tester.takeException(), isFlutterError);
+  });
 }