Fix NSME in `type_annotate_public_apis` (#151).

lib/src/rules/type_annotate_public_apis.dart

 // Copyright (c) 2015, 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.type_annotate_public_apis;
 
 import 'package:analyzer/src/generated/ast.dart';
+import 'package:linter/src/ast.dart';
 import 'package:linter/src/linter.dart';
-import 'package:linter/src/ast.dart';
 
 const desc = r'Type annotate public APIs.';
 
 const details = r'''
 From [effective dart] (https://www.dartlang.org/effective-dart/design/#do-type-annotate-public-apis):
 
 **DO** type annotate public APIs.
 
 Type annotations are important documentation for how a library should be used.
 Annotating the parameter and return types of public methods and functions helps
@@ skipping 34 matching lines @@
             name: 'type_annotate_public_apis',
             description: desc,
             details: details,
             group: Group.style);
 
   @override
   AstVisitor getVisitor() => new Visitor(this);
 }
 
 class Visitor extends SimpleAstVisitor {
+  _VisitorHelper v;
   final LintRule rule;
-  _VisitoHelper v;
   Visitor(this.rule) {
-    v = new _VisitoHelper(rule);
+    v = new _VisitorHelper(rule);
   }
 
   @override
   visitFieldDeclaration(FieldDeclaration node) {
     if (node.fields.type == null) {
       node.fields.accept(v);
     }
   }
 
   @override
-  visitTopLevelVariableDeclaration(TopLevelVariableDeclaration node) {
-    if (node.variables.type == null) {
-      node.variables.accept(v);
-    }
-  }
-
-  @override
   visitFunctionDeclaration(FunctionDeclaration node) {
     if (!isPrivate(node.name)) {
-      if (node.returnType == null) {
+      if (node.returnType == null && !node.isSetter) {
         rule.reportLint(node.name);
       } else {
-        node.functionExpression.parameters.accept(v);
-      }
-    }
-  }
-
-  @override
-  visitMethodDeclaration(MethodDeclaration node) {
-    if (!isPrivate(node.name)) {
-      if (node.returnType == null) {
-        rule.reportLint(node.name);
-      } else {
-        node.parameters.accept(v);
+        node.functionExpression.parameters?.accept(v);
       }
     }
   }
 
   @override
   visitFunctionTypeAlias(FunctionTypeAlias node) {
     if (node.returnType == null) {
       rule.reportLint(node.name);
     } else {
       node.parameters.accept(v);
     }
   }
+
+  @override
+  visitMethodDeclaration(MethodDeclaration node) {
+    if (!isPrivate(node.name)) {
+      if (node.returnType == null && !node.isSetter) {
+        rule.reportLint(node.name);
+      } else {
+        node.parameters?.accept(v);
+      }
+    }
+  }
+
+  @override
+  visitTopLevelVariableDeclaration(TopLevelVariableDeclaration node) {
+    if (node.variables.type == null) {
+      node.variables.accept(v);
+    }
+  }
 }
 
-class _VisitoHelper extends RecursiveAstVisitor {
+class _VisitorHelper extends RecursiveAstVisitor {
   final LintRule rule;
-  _VisitoHelper(this.rule);
+  _VisitorHelper(this.rule);
+
+  @override
+  visitSimpleFormalParameter(SimpleFormalParameter param) {
+    if (param.type == null) {
+      rule.reportLint(param);
+    }
+  }
 
   @override
   visitVariableDeclaration(VariableDeclaration node) {
     if (!isPrivate(node.name)) {
       rule.reportLint(node.name);
     }
   }
-
-  @override
-  visitSimpleFormalParameter(SimpleFormalParameter param) {
-    if (param.type == null) {
-      rule.reportLint(param);
-    }
-  }
 }