Add width constraints for FlexibleSpaceBar.title in its expanded state, so that overflow of long titles can be handled (#51335)

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

@@ -394,7 +394,15 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
                   alignment: titleAlignment,
                   child: DefaultTextStyle(
                     style: titleStyle,
+                    child: LayoutBuilder(
+                      builder: (BuildContext context, BoxConstraints constraints) {
+                        return Container(
+                          width: constraints.maxWidth / scaleValue,
+                          alignment: titleAlignment,
                           child: title,
+                        );
+                      }
+                    ),
                   ),
                 ),
               ),

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -117,6 +117,57 @@ void main() {
     expect(clipRect.size.height, minExtent);
   });
 
+  // This is a regression test for https://github.com/flutter/flutter/issues/14227
+  testWidgets('FlexibleSpaceBar sets width constraints for the title', (WidgetTester tester) async {
+    const double titleFontSize = 20.0;
+    const double height = 300.0;
+    double width;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              width = MediaQuery.of(context).size.width;
+              return CustomScrollView(
+                slivers: <Widget>[
+                  SliverAppBar(
+                    expandedHeight: height,
+                    pinned: true,
+                    stretch: true,
+                    flexibleSpace: FlexibleSpaceBar(
+                      titlePadding: EdgeInsets.zero,
+                      title: Text(
+                        'X' * 2000,
+                        maxLines: 1,
+                        overflow: TextOverflow.ellipsis,
+                        style: const TextStyle(fontSize: titleFontSize),
+                      ),
+                      centerTitle: false,
+                    ),
+                  ),
+                ],
+              );
+            }
+          ),
+        ),
+      ),
+    );
+
+    // The title is scaled and transformed to be 1.5 times bigger, when the
+    // FlexibleSpaceBar is fully expanded, thus we expect the width to be
+    // 1.5 times smaller than the full width. The height of the text is the same
+    // as the font size, with 10 dps bottom margin.
+    expect(
+      tester.getRect(find.byType(Text)),
+      Rect.fromLTRB(
+        0,
+        height - titleFontSize - 10,
+        (width / 1.5).floorToDouble(),
+        height - 10,
+      ),
+    );
+  });
+
   testWidgets('FlexibleSpaceBar test titlePadding defaults', (WidgetTester tester) async {
     Widget buildFrame(TargetPlatform platform, bool centerTitle) {
       return MaterialApp(