Add barrierDismissable argument to showDialog (#8397)

In some situations, dialogs don't want the barrier to be dismissable.

Fixes #8354

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

@@ -316,9 +316,12 @@ class SimpleDialog extends StatelessWidget {
 
 class _DialogRoute<T> extends PopupRoute<T> {
   _DialogRoute({
-    this.child,
+    @required this.theme,
-    this.theme,
+    bool barrierDismissable: true,
-  });
+    @required this.child,
+  }) : _barrierDismissable = barrierDismissable {
+    assert(barrierDismissable != null);
+  }
 
   final Widget child;
   final ThemeData theme;
@@ -327,7 +330,8 @@ class _DialogRoute<T> extends PopupRoute<T> {
   Duration get transitionDuration => const Duration(milliseconds: 150);
 
   @override
-  bool get barrierDismissable => true;
+  bool get barrierDismissable => _barrierDismissable;
+  final bool _barrierDismissable;
 
   @override
   Color get barrierColor => Colors.black54;
@@ -364,10 +368,12 @@ class _DialogRoute<T> extends PopupRoute<T> {
 ///  * <https://material.google.com/components/dialogs.html>
 Future<T> showDialog<T>({
   @required BuildContext context,
-  @required Widget child
+  bool barrierDismissable: true,
+  @required Widget child,
 }) {
   return Navigator.push(context, new _DialogRoute<T>(
     child: child,
     theme: Theme.of(context, shadowThemeOnly: true),
+    barrierDismissable: barrierDismissable,
   ));
 }

packages/flutter/test/material/dialog_test.dart

@@ -137,4 +137,61 @@ void main() {
 
     expect(await result, equals(42));
   });
+
+  testWidgets('Barrier dismissable', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Material(
+          child: new Center(
+            child: new RaisedButton(
+              onPressed: null,
+              child: new Text('Go'),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    BuildContext context = tester.element(find.text('Go'));
+
+    showDialog<Null>(
+      context: context,
+      child: new Container(
+        width: 100.0,
+        height: 100.0,
+        alignment: FractionalOffset.center,
+        child: new Text('Dialog1'),
+      ),
+    );
+
+    await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
+    expect(find.text('Dialog1'), findsOneWidget);
+
+    // Tap on the barrier.
+    await tester.tapAt(const Point(10.0, 10.0));
+
+    await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
+    expect(find.text('Dialog1'), findsNothing);
+
+    showDialog<Null>(
+      context: context,
+      barrierDismissable: false,
+      child: new Container(
+        width: 100.0,
+        height: 100.0,
+        alignment: FractionalOffset.center,
+        child: new Text('Dialog2'),
+      ),
+    );
+
+    await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
+    expect(find.text('Dialog2'), findsOneWidget);
+
+    // Tap on the barrier, which shouldn't do anything this time.
+    await tester.tapAt(const Point(10.0, 10.0));
+
+    await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
+    expect(find.text('Dialog2'), findsOneWidget);
+
+  });
 }