Lint to check for declared return types (#146).

BUG=146
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//1414793007 .

lib/src/rules.dart

@@ -8,6 +8,7 @@ import 'dart:collection';
 
 import 'package:linter/src/config.dart';
 import 'package:linter/src/linter.dart';
+import 'package:linter/src/rules/always_declare_return_types.dart';
 import 'package:linter/src/rules/always_specify_types.dart';
 import 'package:linter/src/rules/camel_case_types.dart';
 import 'package:linter/src/rules/constant_identifier_names.dart';
@@ -27,6 +28,7 @@ import 'package:linter/src/rules/unnecessary_brace_in_string_interp.dart';
 import 'package:linter/src/rules/unnecessary_getters_setters.dart';
 
 final Registry ruleRegistry = new Registry()
+  ..register(new AlwaysDeclareReturnTypes())
   ..register(new AlwaysSpecifyTypes())
   ..register(new CamelCaseTypes())
   ..register(new ConstantIdentifierNames())

lib/src/rules/always_declare_return_types.dart

@@ -0,0 +1,84 @@
+// 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.always_declare_return_types;
+
+import 'package:analyzer/src/generated/ast.dart'
+    show
+        AstVisitor,
+        FunctionDeclaration,
+        FunctionTypeAlias,
+        MethodDeclaration,
+        SimpleAstVisitor;
+import 'package:linter/src/linter.dart';
+
+const desc = 'Declare method return types.';
+
+const details = '''
+**DO** declare method return types.
+
+When declaring a method or function *always* specify a return type.
+
+**BAD:**
+```
+main() { }
+
+_bar() => new _Foo();
+
+class _Foo {
+  _foo() => 42;
+}
+```
+
+**GOOD:**
+```
+void main() { }
+
+_Foo _bar() => new _Foo();
+
+class _Foo {
+  int _foo() => 42;
+}
+
+typedef bool predicate(Object o);
+```
+''';
+
+class AlwaysDeclareReturnTypes extends LintRule {
+  AlwaysDeclareReturnTypes()
+      : super(
+            name: 'always_declare_return_types',
+            description: desc,
+            details: details,
+            group: Group.style);
+
+  @override
+  AstVisitor getVisitor() => new Visitor(this);
+}
+
+class Visitor extends SimpleAstVisitor {
+  final LintRule rule;
+  Visitor(this.rule);
+
+  @override
+  visitFunctionDeclaration(FunctionDeclaration node) {
+    if (node.returnType == null) {
+      rule.reportLint(node.name);
+    }
+  }
+
+  @override
+  visitFunctionTypeAlias(FunctionTypeAlias node) {
+    if (node.returnType == null) {
+      rule.reportLint(node.name);
+    }
+  }
+
+  @override
+  visitMethodDeclaration(MethodDeclaration node) {
+    if (node.returnType == null) {
+      rule.reportLint(node.name);
+    }
+  }
+}

test/_data/p5/p5.dart

@@ -4,6 +4,6 @@
 
 import 'package:p6/p6_lib.dart';
 
-main() {
+void main() {
   p6_lib();
 }

test/rules/always_declare_return_types.dart

@@ -0,0 +1,23 @@
+// 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.
+
+main() { } //LINT
+
+bar() => new _Foo(); //LINT
+
+class _Foo {
+  _foo() => 42; //LINT
+}
+
+typedef bad(int x); //LINT
+
+typedef bool predicate(Object o);
+
+void main2() { }
+
+_Foo bar2() => new _Foo();
+
+class _Foo2 {
+  int _foo() => 42;
+}