Ensure constructors are sorted first (#186).

BUG=
R=scheglov@google.com

Review URL: https://codereview.chromium.org//1706933002 .

lib/src/rules.dart

@@ -29,6 +29,7 @@ import 'package:linter/src/rules/package_prefixed_library_names.dart';
 import 'package:linter/src/rules/prefer_is_not_empty.dart';
 import 'package:linter/src/rules/pub/package_names.dart';
 import 'package:linter/src/rules/slash_for_doc_comments.dart';
+import 'package:linter/src/rules/sort_constructors_first.dart';
 import 'package:linter/src/rules/super_goes_last.dart';
 import 'package:linter/src/rules/type_annotate_public_apis.dart';
 import 'package:linter/src/rules/type_init_formals.dart';
@@ -57,6 +58,7 @@ final Registry ruleRegistry = new Registry()
   ..register(new PackagePrefixedLibraryNames())
   ..register(new PubPackageNames())
   ..register(new SlashForDocComments())
+  ..register(new SortConstructorsFirst())
   ..register(new SuperGoesLast())
   ..register(new TypeInitFormals())
   ..register(new TypeAnnotatePublicApis())

lib/src/rules/sort_constructors_first.dart

@@ -0,0 +1,67 @@
+// 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.sort_constructors_first;
+
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:linter/src/linter.dart';
+
+const desc = r'Sort constructor declarations before method declarations.';
+
+const details = r'''
+
+**DO** sort constructor declarations before method declarations.
+
+**GOOD:**
+```
+abstract class Animation<T> {
+  const Animation();
+  void addListener(VoidCallback listener);
+}
+```
+
+**BAD:**
+```
+abstract class Visitor {
+  visitSomething(Something s);
+  Visitor();
+}
+```
+''';
+
+class SortConstructorsFirst extends LintRule {
+  SortConstructorsFirst()
+      : super(
+            name: 'sort_constructors_first',
+            description: desc,
+            details: details,
+            group: Group.style);
+
+  @override
+  AstVisitor getVisitor() => new Visitor(this);
+}
+
+class Visitor extends SimpleAstVisitor {
+  final LintRule rule;
+  Visitor(this.rule);
+
+  @override
+  visitClassDeclaration(ClassDeclaration decl) {
+    // Sort members by offset.
+    List<ClassMember> members = decl.members.toList();
+    members.sort((ClassMember m1, ClassMember m2) => m1.offset - m2.offset);
+
+    bool seenMethod = false;
+    for (ClassMember member in members) {
+      if (member is ConstructorDeclaration) {
+        if (seenMethod) {
+          rule.reportLint(member.returnType);
+        }
+      }
+      if (member is MethodDeclaration) {
+        seenMethod = true;
+      }
+    }
+  }
+}

test/rules/sort_constructors_first.dart

@@ -0,0 +1,21 @@
+// 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.
+
+// test w/ `dart test/util/solo_test.dart sort_constructors_first`
+
+abstract class A {
+  const A();
+  void f();
+}
+
+abstract class B {
+  void f();
+  const B(); //LINT
+}
+
+abstract class C {
+  void f();
+  C(); //LINT
+  C.named(); //LINT
+}