Add version check for doctor --android-licenses (#14249)

flutter doctor --android-licenses requires Android sdkmanager version 26
or later. When sdkmanager is not available (SDKs earlier than v25),
direct users to SDK upgrade instructions. When it's installed but not
v26 or later, emit instructions to run sdkmanager --update.

packages/flutter_tools/lib/src/android/android_workflow.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 
+import '../base/common.dart';
 import '../base/context.dart';
 import '../base/file_system.dart';
 import '../base/io.dart';
@@ -12,6 +13,7 @@ import '../base/platform.dart';
 import '../base/process.dart';
 import '../base/process_manager.dart';
 import '../base/utils.dart';
+import '../base/version.dart';
 import '../doctor.dart';
 import '../globals.dart';
 import 'android_sdk.dart';
@@ -186,6 +188,21 @@ class AndroidWorkflow extends DoctorValidator implements Workflow {
           fs.path.dirname(javaBinary) + os.pathVarSeparator + platform.environment['PATH'];
     }
 
+    if (!processManager.canRun(androidSdk.sdkManagerPath))
+      throwToolExit(
+        'Android sdkmanager tool not found.\n'
+        'Try re-installing or updating your Android SDK,\n'
+        'visit https://flutter.io/setup/#android-setup for detailed instructions.'
+      );
+
+    final Version sdkManagerVersion = new Version.parse(androidSdk.sdkManagerVersion);
+    if (sdkManagerVersion.major < 26)
+      // SDK manager is found, but needs to be updated.
+      throwToolExit(
+        'A newer version of the Android SDK is required. To update, run:\n'
+        '${androidSdk.sdkManagerPath} --update\n'
+      );
+
     final Process process = await runCommand(
       <String>[androidSdk.sdkManagerPath, '--licenses'],
       environment: sdkManagerEnv,