diff --git a/pubspec.yaml b/pubspec.yaml
index 66f7a8a..c4ea892 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -16,7 +16,6 @@ environment:
 # dependencies:
 
 dev_dependencies:
-  collection: ^1.17.0
   http: ^1.0.0
   path: ^1.8.0
   yaml: ^3.1.0
diff --git a/tool/gen_docs.dart b/tool/gen_docs.dart
index 7a33107..d3a5732 100644
--- a/tool/gen_docs.dart
+++ b/tool/gen_docs.dart
@@ -5,98 +5,208 @@
 import 'dart:convert';
 import 'dart:io';
 
-import 'package:collection/collection.dart';
 import 'package:path/path.dart' as p;
 import 'package:yaml/yaml.dart' as yaml;
 import 'package:http/http.dart' as http;
 
+/// Source of truth for linter rules.
+const rulesUrl =
+    'https://raw.githubusercontent.com/dart-lang/site-www/main/src/_data/linter_rules.json';
+
+/// Local cache of linter rules from [rulesUrl].
+///
+/// Relative to package root.
+const rulesCacheFilePath = 'tool/rules.json';
+
+/// Generated rules documentation markdown file.
+///
+/// Relative to package root.
+const rulesMarkdownFilePath = 'rules.md';
+
+/// Fetches the [rulesUrl] JSON description of all lints, saves a cached
+/// summary of the relevant fields in [rulesCacheFilePath], and
+/// updates [rulesMarkdownFilePath] to
+///
+/// Passing any command line argument disables generating documentation,
+/// and makes this tool just verify that the doc is up-to-date with the
+/// [rulesCacheFilePath]. (Which it always should be, since the two
+/// are saved at the same time.)
 void main(List<String> args) async {
-  final justVerify = args.isNotEmpty;
-  final lintRules = <String, List<String>>{};
-
-  final rulesJsonFile = File('tool/rules.json');
-  final rulesUrl =
-      'https://raw.githubusercontent.com/dart-lang/site-www/main/src/_data/linter_rules.json';
-  if (!justVerify) {
-    rulesJsonFile.writeAsStringSync((await http.get(Uri.parse(rulesUrl))).body);
+  final verifyOnly = args.isNotEmpty;
+
+  // Read lint rules.
+  final rulesJson = await _fetchRulesJson(verifyOnly: verifyOnly);
+
+  // Read existing generated Markdown documentation.
+  final rulesMarkdownFile = _packageRelativeFile(rulesMarkdownFilePath);
+  final rulesMarkdownContent = rulesMarkdownFile.readAsStringSync();
+
+  if (verifyOnly) {
+    print('Validating that ${rulesMarkdownFile.path} is up-to-date ...');
+  } else {
+    print('Regenerating ${rulesMarkdownFile.path} ...');
   }
-  final rulesJson = (jsonDecode(rulesJsonFile.readAsStringSync()) as List)
-      .cast<Map<String, dynamic>>();
 
-  final rulesMdFile = File('rules.md');
-  final rulesMdContent = rulesMdFile.readAsStringSync();
+  // Generate new documentation.
+  var newRulesMarkdownContent =
+      _updateMarkdown(rulesMarkdownContent, rulesJson);
+
+  // If no documentation change, all is up-to-date.
+  if (newRulesMarkdownContent == rulesMarkdownContent) {
+    print('${rulesMarkdownFile.path} is up-to-date.');
+    return;
+  }
 
-  if (justVerify) {
-    print('Validating that ${rulesMdFile.path} is up-to-date ...');
+  /// Documentation has changed.
+  if (verifyOnly) {
+    print('${rulesMarkdownFile.path} is not up-to-date (lint tables need to be '
+        'regenerated).');
+    print('');
+    print("Run 'dart tool/gen_docs.dart' to re-generate.");
+    exit(1);
   } else {
-    print('Regenerating ${rulesMdFile.path} ...');
+    // Save [rulesMarkdownFilePath].
+    rulesMarkdownFile.writeAsStringSync(newRulesMarkdownContent);
+    print('Wrote ${rulesMarkdownFile.path}.');
   }
+}
 
-  for (var file in ['lib/core.yaml', 'lib/recommended.yaml']) {
-    var name = p.basenameWithoutExtension(file);
-    lintRules[name] = _parseRules(File(file));
+/// Fetches or load the JSON lint rules.
+///
+/// If [verifyOnly] is `false`, fetches JSON from [rulesUrl],
+/// extracts the needed information, and writes a summary to
+/// [rulesCacheFilePath].
+///
+/// If [verifyOnly] is `true`, only reads the cached data back from
+/// [rulesCacheFilePath].
+Future<Map<String, Map<String, String>>> _fetchRulesJson(
+    {required bool verifyOnly}) async {
+  final rulesJsonFile = _packageRelativeFile(rulesCacheFilePath);
+  if (verifyOnly) {
+    final rulesJsonText = rulesJsonFile.readAsStringSync();
+    return _readJson(rulesJsonText);
   }
+  final rulesJsonText = (await http.get(Uri.parse(rulesUrl))).body;
+  final rulesJson = _readJson(rulesJsonText);
 
-  var newContent = rulesMdContent;
+  // Re-save [rulesJsonFile] file.
+  var newRulesJson = [...rulesJson.values];
+  rulesJsonFile
+      .writeAsStringSync(JsonEncoder.withIndent('  ').convert(newRulesJson));
 
-  for (var ruleSetName in lintRules.keys) {
-    final comment = '<!-- $ruleSetName -->\n';
+  return rulesJson;
+}
 
-    newContent = newContent.replaceRange(
-      newContent.indexOf(comment) + comment.length,
-      newContent.lastIndexOf(comment),
-      _createRuleTable(lintRules[ruleSetName]!, rulesJson),
-    );
-  }
+/// Extracts relevant information from a list of JSON objects.
+///
+/// For each JSON object, includes only the relevant (string-typed) properties,
+/// then creates a map indexed by the `'name'` property of the objects.
+Map<String, Map<String, String>> _readJson(String rulesJsonText) {
+  /// Relevant keys in the JSON information about lints.
+  const relevantKeys = {'name', 'description', 'fixStatus'};
+  final rulesJson = jsonDecode(rulesJsonText) as List<dynamic>;
+  return {
+    for (Map<String, Object?> rule in rulesJson)
+      rule['name'] as String: {
+        for (var key in relevantKeys) key: rule[key] as String
+      }
+  };
+}
 
-  if (justVerify) {
-    if (newContent != rulesMdContent) {
-      print('${rulesMdFile.path} is not up-to-date (lint tables need to be '
-          'regenerated).');
-      print('');
-      print("Run 'dart tool/gen_docs.dart' to re-generate.");
-      exit(1);
-    } else {
-      print('${rulesMdFile.path} is up-to-date.');
-    }
-  } else {
-    // Re-save rules.json.
-    const retainKeys = {'name', 'description', 'fixStatus'};
-    for (var rule in rulesJson) {
-      rule.removeWhere((key, value) => !retainKeys.contains(key));
+/// Inserts new Markdown content for both rule sets into [content].
+///
+/// For both "core" and "recommended" rule sets,
+/// replaces the table between the two `<!-- core -->` and the two
+/// `<!-- recommended -->` markers with a new table generated from
+/// [rulesJson], based on the list of rules in `lib/core.yaml` and
+/// `lib/recommended.yaml`.
+String _updateMarkdown(
+    String content, Map<String, Map<String, String>> rulesJson) {
+  for (var ruleSetName in ['core', 'recommended']) {
+    var ruleFile = _packageRelativeFile(p.join('lib', '$ruleSetName.yaml'));
+    var ruleSet = _parseRules(ruleFile);
+
+    final rangeDelimiter = '<!-- $ruleSetName -->\n';
+    var rangeStart = content.indexOf(rangeDelimiter) + rangeDelimiter.length;
+    var rangeEnd = content.indexOf(rangeDelimiter, rangeStart);
+    if (rangeEnd < 0) {
+      stderr.writeln('Missing "$rangeDelimiter" in $rulesMarkdownFilePath.');
+      continue;
     }
-    rulesJsonFile
-        .writeAsStringSync(JsonEncoder.withIndent('  ').convert(rulesJson));
-
-    // Write out the rules md file.
-    rulesMdFile.writeAsStringSync(newContent);
-    print('Wrote ${rulesMdFile.path}.');
+    content = content.replaceRange(
+        rangeStart, rangeEnd, _createRuleTable(ruleSet, rulesJson));
   }
+  return content;
 }
 
+/// Parses analysis options YAML file, and extracts linter rules.
 List<String> _parseRules(File yamlFile) {
   var yamlData = yaml.loadYaml(yamlFile.readAsStringSync()) as Map;
-  return (yamlData['linter']['rules'] as List).toList().cast<String>();
+  var linterEntry = yamlData['linter'] as Map;
+  return List<String>.from(linterEntry['rules'] as List);
 }
 
+/// Creates markdown source for a table of lint rules.
 String _createRuleTable(
-    List<String> rules, List<Map<String, dynamic>> lintMeta) {
+    List<String> rules, Map<String, Map<String, String>> lintMeta) {
   rules.sort();
 
   final lines = [
     '| Lint Rules | Description | [Fix][] |',
     '| :--------- | :---------- | ------- |',
-    ...rules.map((rule) {
-      final ruleMeta =
-          lintMeta.firstWhereOrNull((meta) => meta['name'] == rule);
-
-      final description = ruleMeta?['description'] as String? ?? '';
-      final hasFix = ruleMeta?['fixStatus'] == 'hasFix';
-      final fixDesc = hasFix ? '✅' : '';
-
-      return '| [`$rule`](https://dart.dev/lints/$rule) | $description | $fixDesc |';
-    }),
+    for (var rule in rules) _createRuleTableRow(rule, lintMeta),
   ];
 
   return '${lines.join('\n')}\n';
 }
+
+/// Creates a line containing the markdown table row for a single lint rule.
+///
+/// Used by [_createRuleTable] for each row in the generated table.
+/// The row should have the same number of entires as the table format,
+/// and should be on a single line with no newline at the end.
+String _createRuleTableRow(
+    String rule, Map<String, Map<String, String>> lintMeta) {
+  final ruleMeta = lintMeta[rule];
+  if (ruleMeta == null) {
+    stderr.writeln("WARNING: Missing rule information for rule: $rule");
+  }
+  final description = ruleMeta?['description'] ?? '';
+  final hasFix = ruleMeta?['fixStatus'] == 'hasFix';
+  final fixDesc = hasFix ? '✅' : '';
+
+  return '| [`$rule`](https://dart.dev/lints/$rule) | '
+      '$description | $fixDesc |';
+}
+
+/// A path relative to the root of this package.
+///
+/// Works independently of the current working directory.
+/// Is based on the location of this script, through [Platform.script].
+File _packageRelativeFile(String packagePath) =>
+    File(p.join(_packageRoot, packagePath));
+
+/// Cached package root used by [_packageRelative].
+final String _packageRoot = _relativePackageRoot();
+
+/// A path to the package root from the current directory.
+///
+/// If the current directory is inside the package, the returned path is
+/// a relative path of a number of `..` segments.
+/// If the current directory is outside of the package, the returned path
+/// may be absolute.
+String _relativePackageRoot() {
+  var rootPath = p.dirname(p.dirname(Platform.script.path));
+  if (p.isRelative(rootPath)) return rootPath;
+  var baseDir = p.current;
+  if (rootPath == baseDir) return '';
+  if (baseDir.startsWith(rootPath)) {
+    var backSteps = <String>[];
+    do {
+      backSteps.add('..');
+      baseDir = p.dirname(baseDir);
+    } while (baseDir != rootPath);
+    return p.joinAll(backSteps);
+  }
+  return rootPath;
+}