Add flutter_lints package (#343)

packages/flutter_lints/CHANGELOG.md

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+## 1.0.0-dev.0
+
+* Initial pre-release

packages/flutter_lints/LICENSE

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+Copyright 2021, The Flutter Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google LLC nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/flutter_lints/README.md

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+[![pub package](https://img.shields.io/pub/v/flutter_lints.svg)](https://pub.dev/packages/flutter_lints)
+
+This package contains a recommended set of lints for [Flutter] apps, packages,
+and plugins to encourage good coding practices.
+
+Lints are surfaced by the [dart analyzer], which statically checks dart code.
+[Dart-enabled IDEs] typically present the issues identified by the analyzer in
+their UI. Alternatively, the analyzer can be invoked manually by running
+`flutter analyze`.
+
+This package is built on top of Dart's `recommended.yaml` set of lints from
+[package:lints].
+
+## Usage
+
+Flutter apps, packages, and plugins created with `flutter create` starting with
+Flutter version 2.xx are already set up to use the lints defined in this
+package. Entities created before that version can use these lints by following
+these instructions:
+
+1. Depend on this package as a **dev_dependency** by running
+  `flutter pub add --dev flutter_lints`.
+2. Create an `analyzer_options.yaml` file at the root of the package (alongside
+   the `pubspec.yaml` file) and `include: package:flutter_lints/flutter.yaml`
+   from it.
+
+Example `analyzer_options.yaml` file:
+
+```yaml
+# This file configures the analyzer, which statically analyzes Dart code to
+# check for errors, warnings, and lints.
+#
+# The issues identified by the analyzer are surfaced in the UI of Dart-enabled
+# IDEs (https://dart.dev/tools#ides-and-editors). The analyzer can also be
+# invoked from the command line by running `flutter analyze`.
+
+# The following line activates a set of recommended lints for Flutter apps,
+# packages, and plugins designed to encourage good coding practices.
+include: package:flutter_lints/flutter.yaml
+
+linter:
+  # The lint rules applied to this project can be customized in the
+  # section below to disable rules from the `package:flutter_lints/flutter.yaml`
+  # included above or to enable additional rules. A list of all available lints
+  # and their documentation is published at
+  # https://dart-lang.github.io/linter/lints/index.html.
+  #
+  # Instead of disabling a lint rule for the entire project in the
+  # section below, it can also be suppressed for a single line of code
+  # or a specific dart file by using the `// ignore: name_of_lint` and
+  # `// ignore_for_file: name_of_lint` syntax on the line or in the file
+  # producing the lint.
+  rules:
+    # avoid_print: false  # Uncomment to disable the `avoid_print` rule
+    # prefer_single_quotes: true  # Uncomment to enable the `prefer_single_quotes` rule
+
+# Additional information about this file can be found at
+# https://dart.dev/guides/language/analysis-options
+```
+
+[Flutter]: https://flutter.dev
+[dart analyzer]: https://dart.dev/guides/language/analysis-options
+[Dart-enabled IDEs]: https://dart.dev/tools#ides-and-editors
+[package:lints]: https://pub.dev/packages/lints

packages/flutter_lints/lib/flutter.yaml

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+# Recommended lints for Flutter apps, packages, and plugins.
+
+include: package:lints/recommended.yaml
+
+linter:
+  rules:
+    - avoid_print
+    - avoid_unnecessary_containers
+    - avoid_web_libraries_in_flutter
+    - no_logic_in_create_state
+    - prefer_const_constructors
+    - prefer_const_constructors_in_immutables
+    - prefer_const_declarations
+    - prefer_const_literals_to_create_immutables
+    - sized_box_for_whitespace
+    - use_full_hex_values_for_flutter_colors
+    - use_key_in_widget_constructors

packages/flutter_lints/pubspec.yaml

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+name: flutter_lints
+description: Recommended lints for Flutter apps, packages, and plugins.
+version: 1.0.0-dev.0
+homepage: https://github.com/flutter/packages/tree/master/packages/flutter_lints
+
+environment:
+  sdk: '>=2.12.0 <3.0.0'
+
+dependencies:
+  lints: ^1.0.0-dev.0
+  # Code is not allowed in this package. Do not add any dependencies or dev_dependencies.

packages/flutter_lints/run_tests.sh

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+#!/bin/bash
+
+filecount=`find . -name '*.dart' | wc -l`
+if [ $filecount -ne 0 ]
+then
+  echo 'Dart sources are not allowed in this package:'
+  find . -name '*.dart'
+  exit 1
+fi