[flutter_tools] Throw a tool exit when samples fetch fails (#52355)

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

@@ -239,13 +239,21 @@ class CreateCommand extends FlutterCommand {
     }
 
     final Uri snippetsUri = Uri.https(_snippetsHost, 'snippets/$sampleId.dart');
-    return utf8.decode(await _net.fetchUrl(snippetsUri));
+    final List<int> data = await _net.fetchUrl(snippetsUri);
+    if (data == null || data.isEmpty) {
+      return null;
+    }
+    return utf8.decode(data);
   }
 
   /// Fetches the samples index file from the Flutter docs website.
   Future<String> _fetchSamplesIndexFromServer() async {
     final Uri snippetsUri = Uri.https(_snippetsHost, 'snippets/index.json');
-    return utf8.decode(await _net.fetchUrl(snippetsUri, maxAttempts: 2));
+    final List<int> data = await _net.fetchUrl(snippetsUri, maxAttempts: 2);
+    if (data == null || data.isEmpty) {
+      return null;
+    }
+    return utf8.decode(data);
   }
 
   /// Fetches the samples index file from the server and writes it to
@@ -259,8 +267,7 @@ class CreateCommand extends FlutterCommand {
       final String samplesJson = await _fetchSamplesIndexFromServer();
       if (samplesJson == null) {
         throwToolExit('Unable to download samples', exitCode: 2);
-      }
-      else {
+      } else {
         outputFile.writeAsStringSync(samplesJson);
         globals.printStatus('Wrote samples JSON to "$outputFilePath"');
       }

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -1283,6 +1283,28 @@ void main() {
     HttpClientFactory: () =>
         () => MockHttpClient(200, result: samplesIndexJson),
   });
+
+  testUsingContext('Throws tool exit on empty samples index', () async {
+    final String outputFile = globals.fs.path.join(tempDir.path, 'flutter_samples.json');
+    final CreateCommand command = CreateCommand();
+    final CommandRunner<void> runner = createTestCommandRunner(command);
+    final List<String> args = <String>[
+      'create',
+      '--list-samples',
+      outputFile,
+    ];
+
+    await expectLater(
+      runner.run(args),
+      throwsToolExit(
+        exitCode: 2,
+        message: 'Unable to download samples',
+    ));
+  }, overrides: <Type, Generator>{
+    HttpClientFactory: () =>
+        () => MockHttpClient(200, result: ''),
+  });
+
   testUsingContext('provides an error to the user if samples json download fails', () async {
     final String outputFile = globals.fs.path.join(tempDir.path, 'flutter_samples.json');
     final CreateCommand command = CreateCommand();