[flutter_markdown][ci] Manual roll that specifies the correct DropdownMenu import  (#2898)

* Manual roll with dropdown class

* update CHANGELOG
diff --git a/.ci/flutter_master.version b/.ci/flutter_master.version
index 9194851..10b085a 100644
--- a/.ci/flutter_master.version
+++ b/.ci/flutter_master.version
@@ -1 +1 @@
-17c1dbc47378da46bb4472dd77a6b12158fec48e
+d52e2de5bf16249f3a44319ae2807169f9b0b13a
diff --git a/packages/flutter_markdown/CHANGELOG.md b/packages/flutter_markdown/CHANGELOG.md
index 518e26a..477eb94 100644
--- a/packages/flutter_markdown/CHANGELOG.md
+++ b/packages/flutter_markdown/CHANGELOG.md
@@ -1,3 +1,7 @@
+## NEXT
+
+* Updates the example to specify the import for `DropdownMenu`.
+
 ## 0.6.13
 
 * Support changes in the latest `package:markdown`.
diff --git a/packages/flutter_markdown/example/lib/demos/basic_markdown_demo.dart b/packages/flutter_markdown/example/lib/demos/basic_markdown_demo.dart
index 58553ec..49fce54 100644
--- a/packages/flutter_markdown/example/lib/demos/basic_markdown_demo.dart
+++ b/packages/flutter_markdown/example/lib/demos/basic_markdown_demo.dart
@@ -6,7 +6,7 @@
 import 'package:flutter/services.dart';
 import 'package:flutter_markdown/flutter_markdown.dart';
 
-import '../shared/dropdown_menu.dart';
+import '../shared/dropdown_menu.dart' as dropdown;
 import '../shared/markdown_demo_widget.dart';
 import '../shared/markdown_extensions.dart';
 
@@ -85,7 +85,7 @@
         if (snapshot.connectionState == ConnectionState.done) {
           return Column(
             children: <Widget>[
-              DropdownMenu<MarkdownExtensionSet>(
+              dropdown.DropdownMenu<MarkdownExtensionSet>(
                 items: _menuItems,
                 label: 'Extension Set:',
                 initialValue: _extensionSet,
diff --git a/packages/flutter_markdown/example/lib/demos/markdown_body_shrink_wrap_demo.dart b/packages/flutter_markdown/example/lib/demos/markdown_body_shrink_wrap_demo.dart
index d2de056..a058da2 100644
--- a/packages/flutter_markdown/example/lib/demos/markdown_body_shrink_wrap_demo.dart
+++ b/packages/flutter_markdown/example/lib/demos/markdown_body_shrink_wrap_demo.dart
@@ -7,7 +7,7 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter_markdown/flutter_markdown.dart';
-import '../shared/dropdown_menu.dart';
+import '../shared/dropdown_menu.dart' as dropdown;
 import '../shared/markdown_demo_widget.dart';
 
 // ignore_for_file: public_member_api_docs
@@ -67,7 +67,7 @@
   Widget build(BuildContext context) {
     return Column(
       children: <Widget>[
-        DropdownMenu<bool>(
+        dropdown.DropdownMenu<bool>(
           items: _shrinkWrapMenuItems,
           label: 'Shrink wrap:',
           initialValue: _shrinkWrap,
diff --git a/packages/flutter_markdown/example/lib/demos/wrap_alignment_demo.dart b/packages/flutter_markdown/example/lib/demos/wrap_alignment_demo.dart
index da4cbc8..b86f825 100644
--- a/packages/flutter_markdown/example/lib/demos/wrap_alignment_demo.dart
+++ b/packages/flutter_markdown/example/lib/demos/wrap_alignment_demo.dart
@@ -6,7 +6,7 @@
 import 'package:flutter/services.dart';
 import 'package:flutter_markdown/flutter_markdown.dart';
 import 'package:markdown/markdown.dart' as md;
-import '../shared/dropdown_menu.dart';
+import '../shared/dropdown_menu.dart' as dropdown;
 import '../shared/markdown_demo_widget.dart';
 import '../shared/markdown_extensions.dart';
 
@@ -79,7 +79,7 @@
         if (snapshot.connectionState == ConnectionState.done) {
           return Column(
             children: <Widget>[
-              DropdownMenu<WrapAlignment>(
+              dropdown.DropdownMenu<WrapAlignment>(
                 items: _wrapAlignmentMenuItems,
                 label: 'Wrap Alignment:',
                 initialValue: _wrapAlignment,
@@ -91,7 +91,7 @@
                   }
                 },
               ),
-              DropdownMenu<double>(
+              dropdown.DropdownMenu<double>(
                 items: _blockSpacingMenuItems,
                 label: 'Block Spacing:',
                 initialValue: _blockSpacing,