Fix missing_return hint in _getInferredMixinType

Change-Id: Ie9aa833727f40a38bd6a69e027c2bbaf8d6dec72
Reviewed-on: https://dart-review.googlesource.com/75988
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index b5f0cb0..1e780ba 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -7868,6 +7868,7 @@
     for (var candidateMixin in classElement.mixins) {
       if (candidateMixin.element == mixinElement) return candidateMixin;
     }
+    return null; // Not found
   }
 
   /// The number of type arguments in the given [typeName] does not match the