switching channel prompts to run flutter upgrade (#49511)

packages/flutter_tools/lib/src/commands/channel.dart

diff --git a/packages/flutter_tools/lib/src/commands/channel.dart b/packages/flutter_tools/lib/src/commands/channel.dart
index a91fa9d..b7f91db 100644
--- a/packages/flutter_tools/lib/src/commands/channel.dart
+++ b/packages/flutter_tools/lib/src/commands/channel.dart
@@ -116,7 +116,7 @@
     }
   }
 
-  Future<void> _switchChannel(String branchName) {
+  Future<void> _switchChannel(String branchName) async {
     globals.printStatus("Switching to flutter channel '$branchName'...");
     if (FlutterVersion.obsoleteBranches.containsKey(branchName)) {
       final String alternative = FlutterVersion.obsoleteBranches[branchName];
@@ -124,7 +124,9 @@
     } else if (!FlutterVersion.officialChannels.contains(branchName)) {
       globals.printStatus('This is not an official channel. For a list of available channels, try "flutter channel".');
     }
-    return _checkout(branchName);
+    await _checkout(branchName);
+    globals.printStatus("Successfully switched to flutter channel '$branchName'.");
+    globals.printStatus("To ensure that you're on the latest build from this channel, run 'flutter upgrade'");
   }
 
   static Future<void> upgradeChannel() async {

packages/flutter_tools/test/general.shard/channel_test.dart

diff --git a/packages/flutter_tools/test/general.shard/channel_test.dart b/packages/flutter_tools/test/general.shard/channel_test.dart
index 75fee2b..850ddbf 100644
--- a/packages/flutter_tools/test/general.shard/channel_test.dart
+++ b/packages/flutter_tools/test/general.shard/channel_test.dart
@@ -253,6 +253,51 @@
       ProcessManager: () => mockProcessManager,
     });
 
+    testUsingContext('switching channels prompts to run flutter upgrade', () async {
+      when(mockProcessManager.start(
+        <String>['git', 'fetch'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).thenAnswer((_) => Future<Process>.value(createMockProcess()));
+      when(mockProcessManager.start(
+        <String>['git', 'show-ref', '--verify', '--quiet', 'refs/heads/beta'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).thenAnswer((_) => Future<Process>.value(createMockProcess()));
+      when(mockProcessManager.start(
+        <String>['git', 'checkout', 'beta', '--'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).thenAnswer((_) => Future<Process>.value(createMockProcess()));
+
+      final ChannelCommand command = ChannelCommand();
+      final CommandRunner<void> runner = createTestCommandRunner(command);
+      await runner.run(<String>['channel', 'beta']);
+
+      verify(mockProcessManager.start(
+        <String>['git', 'fetch'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).called(1);
+      verify(mockProcessManager.start(
+        <String>['git', 'show-ref', '--verify', '--quiet', 'refs/heads/beta'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).called(1);
+      verify(mockProcessManager.start(
+        <String>['git', 'checkout', 'beta', '--'],
+        workingDirectory: anyNamed('workingDirectory'),
+        environment: anyNamed('environment'),
+      )).called(1);
+
+      expect(testLogger.statusText, contains("Successfully switched to flutter channel 'beta'."));
+      expect(testLogger.statusText, contains("To ensure that you're on the latest build from this channel, run 'flutter upgrade'"));
+      expect(testLogger.errorText, hasLength(0));
+    }, overrides: <Type, Generator>{
+      FileSystem: () => MemoryFileSystem(),
+      ProcessManager: () => mockProcessManager,
+    });
+
     // This verifies that bug https://github.com/flutter/flutter/issues/21134
     // doesn't return.
     testUsingContext('removes version stamp file when switching channels', () async {