Issue 41065. Remove unnecessary null workaround.

Bug: #41065
Change-Id: I328b334fbdf4d4c77453cca56ae0c42ae43e8ee3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155822
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analyzer/lib/src/dart/resolver/invocation_inference_helper.dart

@@ -45,8 +45,7 @@ class InvocationInferenceHelper {
   /// type that is being invoked.
   DartType computeInvokeReturnType(DartType type) {
     if (type is FunctionType) {
-      // TODO(scheglov) https://github.com/dart-lang/sdk/issues/41065
-      return type.returnType ?? DynamicTypeImpl.instance;
+      return type.returnType;
     } else {
       return DynamicTypeImpl.instance;
     }