Fix crash on flutter update-packages (#28922)

When running git update-packages, if the goldens repo is dirty, the
flutter tool currently crashes and logs the failure from git pull.

This adds a check for a dirty git client and emits a friendlier error
message in the case where it's not clean, and avoid the crash by
catching the NonZeroExitCode exception and rethrowing as toolExit
instead.

Fixes: https://github.com/flutter/flutter/issues/28915

packages/flutter_goldens_client/lib/client.dart

@@ -80,6 +80,7 @@ class GoldensClient {
           if (currentCommit == null) {
             await _initRepository();
           }
+          await _checkCanSync();
           await _syncTo(goldensCommit);
         }
       } finally {
@@ -117,6 +118,23 @@ class GoldensClient {
     );
   }
 
+  Future<void> _checkCanSync() async {
+    final io.ProcessResult result = await process.run(
+      <String>['git', 'status', '--porcelain'],
+      workingDirectory: repositoryRoot.path,
+    );
+    if (result.stdout.trim().isNotEmpty) {
+      final StringBuffer buf = StringBuffer();
+      buf
+        ..writeln('flutter_goldens git checkout at ${repositoryRoot.path} has local changes and cannot be synced.')
+        ..writeln('To reset your client to a clean state, and lose any local golden test changes:')
+        ..writeln('cd ${repositoryRoot.path}')
+        ..writeln('git reset --hard HEAD')
+        ..writeln('git clean -x -d -f -f');
+      throw NonZeroExitCode(1, buf.toString());
+    }
+  }
+
   Future<void> _syncTo(String commit) async {
     await _runCommands(
       <String>[
@@ -163,7 +181,7 @@ class NonZeroExitCode implements Exception {
   /// The first argument must be non-zero.
   const NonZeroExitCode(this.exitCode, this.stderr) : assert(exitCode != 0);
 
-  /// The code that the process will signal to th eoperating system.
+  /// The code that the process will signal to the operating system.
   ///
   /// By definiton, this is not zero.
   final int exitCode;

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

@@ -132,8 +132,12 @@ class UpdatePackagesCommand extends FlutterCommand {
     // package that is in the goldens repository. We need to make sure that the goldens
     // repository is cloned locally before we verify or update pubspecs.
     printStatus('Cloning goldens repository...');
-    final GoldensClient goldensClient = GoldensClient();
-    await goldensClient.prepare();
+    try {
+      final GoldensClient goldensClient = GoldensClient();
+      await goldensClient.prepare();
+    } on NonZeroExitCode catch (e) {
+      throwToolExit(e.stderr, exitCode: e.exitCode);
+    }
 
     if (isVerifyOnly) {
       bool needsUpdate = false;