[analyzer] Fix runtime error when creating SDK manager

Caused by a ternary operator with differently typed branches.

Change-Id: I5f9cfc28c274c125db0a999237f81dc3523896cc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104240
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

pkg/analyzer/lib/src/command_line/arguments.dart

@@ -128,7 +128,7 @@ DartSdkManager createDartSdkManager(
         return !context.isWithin(sdkPath, sourcePath);
       });
   return new DartSdkManager(
-      sdkPath ?? FolderBasedDartSdk.defaultSdkDirectory(resourceProvider),
+      sdkPath ?? FolderBasedDartSdk.defaultSdkDirectory(resourceProvider)?.path,
       canUseSummaries);
 }
 

pkg/analyzer/test/src/command_line/arguments_test.dart

@@ -84,7 +84,7 @@ class ArgumentsTest with ResourceProviderMixin {
         createDartSdkManager(resourceProvider, false, result);
     expect(manager, isNotNull);
     expect(manager.defaultSdkDirectory,
-        FolderBasedDartSdk.defaultSdkDirectory(resourceProvider));
+        FolderBasedDartSdk.defaultSdkDirectory(resourceProvider)?.path);
     expect(manager.canUseSummaries, false);
   }
 
@@ -96,8 +96,9 @@ class ArgumentsTest with ResourceProviderMixin {
     DartSdkManager manager =
         createDartSdkManager(resourceProvider, true, result);
     expect(manager, isNotNull);
+
     expect(manager.defaultSdkDirectory,
-        FolderBasedDartSdk.defaultSdkDirectory(resourceProvider));
+        FolderBasedDartSdk.defaultSdkDirectory(resourceProvider)?.path);
     expect(manager.canUseSummaries, true);
   }