Fix exception in linter (issue 315)

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 51 matching lines @@
   @override
   AstVisitor getVisitor() => _visitor;
 }
 
 class _Visitor extends SimpleAstVisitor {
   final LintRule rule;
   _Visitor(this.rule);
 
   @override
   void visitFunctionDeclaration(FunctionDeclaration node) {
-    node.functionExpression.parameters.parameters.forEach((e) {
-      if (node.functionExpression.body.isPotentiallyMutatedInScope(e.element)) {
-        rule.reportLint(e);
-      }
-    });
+    FormalParameterList parameters = node.functionExpression.parameters;
+    if (parameters != null) {
+      // Getter do not have formal parameters.
+      parameters.parameters.forEach((e) {
+        if (node.functionExpression.body
+            .isPotentiallyMutatedInScope(e.element)) {
+          rule.reportLint(e);
+        }
+      });
+    }
   }
 
   @override
   void visitMethodDeclaration(MethodDeclaration node) {
     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);
         }
       });
     }
   }
 }