Adding a default title for Title widget (#12015)

Adds a default title to the Title widget, fixes #6802.

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

@@ -84,7 +84,7 @@ class MaterialApp extends StatefulWidget {
   /// The boolean arguments, [routes], and [navigatorObservers], must not be null.
   MaterialApp({ // can't be const because the asserts use methods on Map :-(
     Key key,
-    this.title,
+    this.title: '',
     this.color,
     this.theme,
     this.home,
@@ -103,7 +103,8 @@ class MaterialApp extends StatefulWidget {
     this.checkerboardOffscreenLayers: false,
     this.showSemanticsDebugger: false,
     this.debugShowCheckedModeBanner: true
-  }) : assert(routes != null),
+  }) : assert(title != null),
+       assert(routes != null),
        assert(navigatorObservers != null),
        assert(debugShowMaterialGrid != null),
        assert(showPerformanceOverlay != null),

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

@@ -70,7 +70,7 @@ class WidgetsApp extends StatefulWidget {
     Key key,
     @required this.onGenerateRoute,
     this.onUnknownRoute,
-    this.title,
+    this.title: '',
     this.textStyle,
     @required this.color,
     this.navigatorObservers: const <NavigatorObserver>[],
@@ -86,7 +86,8 @@ class WidgetsApp extends StatefulWidget {
     this.debugShowWidgetInspector: false,
     this.debugShowCheckedModeBanner: true,
     this.inspectorSelectButtonBuilder,
-  }) : assert(onGenerateRoute != null),
+  }) : assert(title != null),
+       assert(onGenerateRoute != null),
        assert(color != null),
        assert(navigatorObservers != null),
        assert(supportedLocales != null && supportedLocales.isNotEmpty),

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

@@ -11,18 +11,26 @@ import 'framework.dart';
 /// A widget that describes this app in the operating system.
 class Title extends StatelessWidget {
   /// Creates a widget that describes this app to the operating system.
+  ///
+  /// [title] will default to the empty string if not supplied.
+  /// [color] must be an opaque color (i.e. color.alpha must be 255 (0xFF)).
+  /// [color] and [child] are required arguments.
   Title({
     Key key,
-    this.title,
+    this.title: '',
-    this.color,
+    @required this.color,
     @required this.child,
-  }) : assert(color == null || color.alpha == 0xFF),
+  }) : assert(title != null),
+       assert(color != null && color.alpha == 0xFF),
        super(key: key);
 
   /// A one-line description of this app for use in the window manager.
+  /// Must not be null.
   final String title;
 
-  /// A color that the window manager should use to identify this app.
+  /// A color that the window manager should use to identify this app.  Must be
+  /// an opaque color (i.e. color.alpha must be 255 (0xFF)), and must not be
+  /// null.
   final Color color;
 
   /// The widget below this widget in the tree.
@@ -42,7 +50,7 @@ class Title extends StatelessWidget {
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder description) {
     super.debugFillProperties(description);
-    description.add(new StringProperty('title', title, defaultValue: null));
+    description.add(new StringProperty('title', title, defaultValue: ''));
     description.add(new DiagnosticsProperty<Color>('color', color, defaultValue: null));
   }
 }

packages/flutter/test/widgets/title_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter/services.dart';
 
 void main() {
   testWidgets('toString control test', (WidgetTester tester) async {
@@ -14,4 +15,52 @@ void main() {
     );
     expect(widget.toString, isNot(throwsException));
   });
+
+  testWidgets('should handle having no title', (WidgetTester tester) async {
+    final Title widget = new Title(
+      child: new Container(),
+      color: const Color(0xFF00FF00),
+    );
+    expect(widget.toString, isNot(throwsException));
+    expect(widget.title, equals(''));
+    expect(widget.color, equals(const Color(0xFF00FF00)));
+  });
+
+  testWidgets('should not allow null title or color', (WidgetTester tester) async {
+    expect(() => new Title(
+      title: null,
+      color: const Color(0xFF00FF00),
+      child: new Container(),
+    ), throwsAssertionError);
+    expect(() => new Title(
+      color: null,
+      child: new Container(),
+    ), throwsAssertionError);
+  });
+
+  testWidgets('should not allow non-opaque color', (WidgetTester tester) async {
+    expect(() => new Title(
+      color: const Color(0),
+      child: new Container(),
+    ), throwsAssertionError);
+  });
+
+  testWidgets('should not pass "null" to setApplicationSwitcherDescription',
+          (WidgetTester tester) async {
+    final List<MethodCall> log = <MethodCall>[];
+
+    SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+      log.add(methodCall);
+    });
+
+    await tester.pumpWidget(new Title(
+      child: new Container(),
+      color: const Color(0xFF00FF00),
+    ));
+
+    expect(log, equals(<MethodCall>[new MethodCall(
+        'SystemChrome.setApplicationSwitcherDescription',
+        <String, dynamic>{'label': '', 'primaryColor': 4278255360},
+    )]));
+  });
 }