fix analyze command roots validation (#315)

Fixes #282

Previously we were doing a non-null assertion on the wrong variable 🤦‍♂️ . This is now tested as well.

Author: jakemac53

pkgs/dart_mcp_server/CHANGELOG.md

@@ -3,6 +3,7 @@
 - Add `--tools=dart|all` argument to allow enabling only vanilla Dart tools for
   non-flutter projects.
 - Include the device name and target platform in the list_devices tool.
+- Fix analyze tool handling of invalid roots.
 - Fix erroneous SDK version error messages when connecting to a VM Service
   instead of DTD URI.
 

pkgs/dart_mcp_server/lib/src/mixins/analyzer.dart

@@ -275,8 +275,8 @@ base mixin DartAnalyzerSupport
         fileSystem: fileSystem,
       );
 
-      if (validated.errorResult != null) {
-        return errorResult!;
+      if (validated.errorResult case final error?) {
+        return error;
       }
 
       final rootUri = Uri.parse(validated.root!.uri);

pkgs/dart_mcp_server/test/tools/analyzer_test.dart

@@ -277,6 +277,45 @@ void main() {
       );
     });
 
+    test('handles invalid roots list', () async {
+      // We still need a root registered with the server so that the
+      // prerequisites check passes.
+      final example = d.dir('example', [
+        d.file('main.dart', 'void main() => 1;'),
+      ]);
+      await example.create();
+      final exampleRoot = testHarness.rootForPath(example.io.path);
+      testHarness.mcpClient.addRoot(exampleRoot);
+      await pumpEventQueue();
+
+      final request = CallToolRequest(
+        name: analyzeTool.name,
+        arguments: {
+          ParameterNames.roots: [
+            {'root': 'file:///invalid/root'},
+          ],
+        },
+      );
+      final result = await testHarness.callToolWithRetry(
+        request,
+        expectError: true,
+      );
+      expect(result.isError, isTrue);
+      expect(
+        result.content.single,
+        isA<TextContent>().having(
+          (t) => t.text,
+          'text',
+          allOf(
+            contains(
+              'Invalid root file:///invalid/root, must be under one of the registered project roots:',
+            ),
+            contains(example.io.uri.toString()),
+          ),
+        ),
+      );
+    });
+
     test('can analyze files in multiple roots', () async {
       final projectA = d.dir('project_a', [
         d.file('main.dart', 'void main() => 1 + "a";'),