Fix failure when linting getters (issue 27545)

lib/src/rules/parameter_assignments.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library linter.src.rules.parameter_assignments;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:linter/src/linter.dart';
 
@@ skipping 60 matching lines @@
   void visitFunctionDeclaration(FunctionDeclaration node) {
     node.functionExpression.parameters.parameters.forEach((e) {
       if (node.functionExpression.body.isPotentiallyMutatedInScope(e.element)) {
         rule.reportLint(e);
       }
     });
   }
 
   @override
   void visitMethodDeclaration(MethodDeclaration node) {
-    node.parameters.parameters.forEach((e) {
-      if (node.body.isPotentiallyMutatedInScope(e.element)) {
-        rule.reportLint(e);
-      }
-    });
+    FormalParameterList parameterList = node.parameters;
+    if (parameterList != null) {
+      // Getters don't have parameters.
+      parameterList.parameters.forEach((e) {
+        if (node.body.isPotentiallyMutatedInScope(e.element)) {
+          rule.reportLint(e);
+        }
+      });
+    }
   }
 }