Update DropdownButton underline to be customizable (#29138)

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

@@ -599,6 +599,7 @@ class DropdownButton<T> extends StatefulWidget {
     @required this.onChanged,
     this.elevation = 8,
     this.style,
+    this.underline,
     this.icon,
     this.iconDisabledColor,
     this.iconEnabledColor,
@@ -656,6 +657,11 @@ class DropdownButton<T> extends StatefulWidget {
   /// [ThemeData.textTheme] of the current [Theme].
   final TextStyle style;
 
+  /// The widget to use for drawing the drop-down button's underline.
+  ///
+  /// Defaults to a 0.0 width bottom border with color 0xFFBDBDBD.
+  final Widget underline;
+
   /// The widget to use for the drop-down button's icon.
   ///
   /// Defaults to an [Icon] with the [Icons.arrow_drop_down] glyph.
@@ -897,7 +903,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
             left: 0.0,
             right: 0.0,
             bottom: bottom,
-            child: Container(
+            child: widget.underline ?? Container(
               height: 1.0,
               decoration: const BoxDecoration(
                 border: Border(bottom: BorderSide(color: Color(0xFFBDBDBD), width: 0.0))

packages/flutter/test/material/dropdown_test.dart

@@ -39,6 +39,7 @@ Widget buildFrame({
   bool isExpanded = false,
   Widget hint,
   Widget disabledHint,
+  Widget underline,
   List<String> items = menuItems,
   Alignment alignment = Alignment.center,
   TextDirection textDirection = TextDirection.ltr,
@@ -61,6 +62,7 @@ Widget buildFrame({
             iconEnabledColor: iconEnabledColor,
             isDense: isDense,
             isExpanded: isExpanded,
+            underline: underline,
             items: items == null ? null : items.map<DropdownMenuItem<String>>((String item) {
               return DropdownMenuItem<String>(
                 key: ValueKey<String>(item),
@@ -1223,4 +1225,30 @@ void main() {
     await tester.pumpAndSettle();
     expect(selectedIndex, 13);
   });
+
+  testWidgets('Dropdown button will accept widgets as its underline', (
+      WidgetTester tester) async {
+
+    const BoxDecoration decoration = BoxDecoration(
+      border: Border(bottom: BorderSide(color: Color(0xFFCCBB00), width: 4.0)),
+    );
+    const BoxDecoration defaultDecoration = BoxDecoration(
+      border: Border(bottom: BorderSide(color: Color(0xFFBDBDBD), width: 0.0)),
+    );
+
+    final Widget customUnderline = Container(height: 4.0, decoration: decoration);
+    final Key buttonKey = UniqueKey();
+
+    final Finder decoratedBox = find.descendant(
+      of: find.byKey(buttonKey),
+      matching: find.byType(DecoratedBox),
+    );
+
+    await tester.pumpWidget(buildFrame(buttonKey: buttonKey, underline: customUnderline,
+        value: 'two', onChanged: onChanged));
+    expect(tester.widget<DecoratedBox>(decoratedBox).decoration, decoration);
+
+    await tester.pumpWidget(buildFrame(buttonKey: buttonKey, value: 'two', onChanged: onChanged));
+    expect(tester.widget<DecoratedBox>(decoratedBox).decoration, defaultDecoration);
+  });
 }