Fix and test for 'implements' pubspec entry (#4242)

The federated plugin spec calls for implementation packages to include an `implements` entry in the `plugins` section of the `pubspec.yaml` indicating what app-facing package it implements. Most of the described behaviors of the `flutter` tool aren't implemented yet, and the pub.dev features have `default_plugin` as a backstop, so we haven't noticed that they are mostly missing (or in one case, incorrect).

To better future-proof the plugins, and to provide a better example to people looking at our plugins as examples of federation, this adds a CI check to make sure that we are correctly adding it, and fixes all of the missing/incorrect values it turned up.

Fixes https://github.com/flutter/flutter/issues/88222

script/tool/lib/src/pubspec_check_command.dart

@@ -7,6 +7,7 @@ import 'package:git/git.dart';
 import 'package:platform/platform.dart';
 import 'package:pubspec_parse/pubspec_parse.dart';
 
+import 'common/core.dart';
 import 'common/package_looping_command.dart';
 import 'common/process_runner.dart';
 
@@ -65,8 +66,8 @@ class PubspecCheckCommand extends PackageLoopingCommand {
   @override
   Future<PackageResult> runForPackage(Directory package) async {
     final File pubspec = package.childFile('pubspec.yaml');
-    final bool passesCheck = !pubspec.existsSync() ||
-        await _checkPubspec(pubspec, packageName: package.basename);
+    final bool passesCheck =
+        !pubspec.existsSync() || await _checkPubspec(pubspec, package: package);
     if (!passesCheck) {
       return PackageResult.fail();
     }
@@ -75,7 +76,7 @@ class PubspecCheckCommand extends PackageLoopingCommand {
 
   Future<bool> _checkPubspec(
     File pubspecFile, {
-    required String packageName,
+    required Directory package,
   }) async {
     final String contents = pubspecFile.readAsStringSync();
     final Pubspec? pubspec = _tryParsePubspec(contents);
@@ -84,34 +85,43 @@ class PubspecCheckCommand extends PackageLoopingCommand {
     }
 
     final List<String> pubspecLines = contents.split('\n');
-    final List<String> sectionOrder = pubspecLines.contains('  plugin:')
-        ? _majorPluginSections
-        : _majorPackageSections;
+    final bool isPlugin = pubspec.flutter?.containsKey('plugin') ?? false;
+    final List<String> sectionOrder =
+        isPlugin ? _majorPluginSections : _majorPackageSections;
     bool passing = _checkSectionOrder(pubspecLines, sectionOrder);
     if (!passing) {
-      print('${indentation}Major sections should follow standard '
+      printError('${indentation}Major sections should follow standard '
           'repository ordering:');
       final String listIndentation = indentation * 2;
-      print('$listIndentation${sectionOrder.join('\n$listIndentation')}');
+      printError('$listIndentation${sectionOrder.join('\n$listIndentation')}');
     }
 
     if (pubspec.publishTo != 'none') {
       final List<String> repositoryErrors =
-          _checkForRepositoryLinkErrors(pubspec, packageName: packageName);
+          _checkForRepositoryLinkErrors(pubspec, packageName: package.basename);
       if (repositoryErrors.isNotEmpty) {
         for (final String error in repositoryErrors) {
-          print('$indentation$error');
+          printError('$indentation$error');
         }
         passing = false;
       }
 
       if (!_checkIssueLink(pubspec)) {
-        print(
+        printError(
             '${indentation}A package should have an "issue_tracker" link to a '
             'search for open flutter/flutter bugs with the relevant label:\n'
             '${indentation * 2}$_expectedIssueLinkFormat<package label>');
         passing = false;
       }
+
+      if (isPlugin) {
+        final String? error =
+            _checkForImplementsError(pubspec, package: package);
+        if (error != null) {
+          printError('$indentation$error');
+          passing = false;
+        }
+      }
     }
 
     return passing;
@@ -168,4 +178,52 @@ class PubspecCheckCommand extends PackageLoopingCommand {
             .startsWith(_expectedIssueLinkFormat) ==
         true;
   }
+
+  // Validates the "implements" keyword for a plugin, returning an error
+  // string if there are any issues.
+  //
+  // Should only be called on plugin packages.
+  String? _checkForImplementsError(
+    Pubspec pubspec, {
+    required Directory package,
+  }) {
+    if (_isImplementationPackage(package)) {
+      final String? implements =
+          pubspec.flutter!['plugin']!['implements'] as String?;
+      final String expectedImplements = package.parent.basename;
+      if (implements == null) {
+        return 'Missing "implements: $expectedImplements" in "plugin" section.';
+      } else if (implements != expectedImplements) {
+        return 'Expecetd "implements: $expectedImplements"; '
+            'found "implements: $implements".';
+      }
+    }
+    return null;
+  }
+
+  // Returns true if [packageName] appears to be an implementation package
+  // according to repository conventions.
+  bool _isImplementationPackage(Directory package) {
+    // An implementation package should be in a group folder...
+    final Directory parentDir = package.parent;
+    if (parentDir.path == packagesDir.path) {
+      return false;
+    }
+    final String packageName = package.basename;
+    final String parentName = parentDir.basename;
+    // ... whose name is a prefix of the package name.
+    if (!packageName.startsWith(parentName)) {
+      return false;
+    }
+    // A few known package names are not implementation packages; assume
+    // anything else is. (This is done instead of listing known implementation
+    // suffixes to allow for non-standard suffixes; e.g., to put several
+    // platforms in one package for code-sharing purposes.)
+    const Set<String> nonImplementationSuffixes = <String>{
+      '', // App-facing package.
+      '_platform_interface', // Platform interface package.
+    };
+    final String suffix = packageName.substring(parentName.length);
+    return !nonImplementationSuffixes.contains(suffix);
+  }
 }