Fix a few code style nits according to Effective Dart: (#2610)

Fix a few code style nits according to Effective Dart:

* [DON’T use a leading underscore for identifiers that aren’t private.](https://dart.dev/guides/language/effective-dart/style#dont-use-a-leading-underscore-for-identifiers-that-arent-private)
* [DO follow a consistent rule for `var` and `final` on local variables.](https://dart.dev/guides/language/effective-dart/usage#do-follow-a-consistent-rule-for-var-and-final-on-local-variables)

Additionally:

* cleaned up comments to use backticks and references
* use collection-for when declaring a Map, instead of using MapEntries.

Author: srawlins

lib/dartdoc.dart

@@ -171,16 +171,16 @@ class Dartdoc {
   /// thrown if dartdoc fails in an expected way, for example if there is an
   /// analysis error in the code.
   Future<DartdocResults> generateDocsBase() async {
-    var _stopwatch = Stopwatch()..start();
+    var stopwatch = Stopwatch()..start();
     double seconds;
     packageGraph = await packageBuilder.buildPackageGraph();
-    seconds = _stopwatch.elapsedMilliseconds / 1000.0;
+    seconds = stopwatch.elapsedMilliseconds / 1000.0;
     var libs = packageGraph.libraries.length;
     logInfo("Initialized dartdoc with $libs librar${libs == 1 ? 'y' : 'ies'} "
         'in ${seconds.toStringAsFixed(1)} seconds');
-    _stopwatch.reset();
+    stopwatch.reset();
 
-    final generator = this.generator;
+    var generator = this.generator;
     if (generator != null) {
       // Create the out directory.
       if (!outputDir.exists) outputDir.create();
@@ -203,7 +203,7 @@ class Dartdoc {
           "and $errors ${pluralize('error', errors)}.");
     }
 
-    seconds = _stopwatch.elapsedMilliseconds / 1000.0;
+    seconds = stopwatch.elapsedMilliseconds / 1000.0;
     libs = packageGraph.localPublicLibraries.length;
     logInfo("Documented $libs public librar${libs == 1 ? 'y' : 'ies'} "
         'in ${seconds.toStringAsFixed(1)} seconds');

lib/src/model/categorization.dart

@@ -18,23 +18,23 @@ abstract class Categorization implements ModelElement {
   String buildDocumentationAddition(String rawDocs) =>
       _stripAndSetDartdocCategories(rawDocs ??= '');
 
-  /// Parse {@category ...} and related information in API comments, stripping
+  /// Parse `{@category ...}` and related information in API comments, stripping
   /// out that information from the given comments and returning the stripped
   /// version.
   String _stripAndSetDartdocCategories(String rawDocs) {
-    var _categorySet = <String>{};
-    var _subCategorySet = <String>{};
+    var categorySet = <String>{};
+    var subCategorySet = <String>{};
     _hasCategorization = false;
 
     rawDocs = rawDocs.replaceAllMapped(_categoryRegExp, (match) {
       _hasCategorization = true;
       switch (match[1]) {
         case 'category':
         case 'api':
-          _categorySet.add(match[2].trim());
+          categorySet.add(match[2].trim());
           break;
         case 'subCategory':
-          _subCategorySet.add(match[2].trim());
+          subCategorySet.add(match[2].trim());
           break;
         case 'image':
           _image = match[2].trim();
@@ -46,8 +46,8 @@ abstract class Categorization implements ModelElement {
       return '';
     });
 
-    _categoryNames = _categorySet.toList()..sort();
-    _subCategoryNames = _subCategorySet.toList()..sort();
+    _categoryNames = categorySet.toList()..sort();
+    _subCategoryNames = subCategorySet.toList()..sort();
     _image ??= '';
     _samples ??= '';
     return rawDocs;

lib/src/model/library.dart

@@ -4,7 +4,6 @@
 
 import 'dart:collection';
 
-import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type_system.dart';
@@ -73,13 +72,13 @@ class Library extends ModelElement with Categorization, TopLevelContainer {
     var element = resolvedLibrary.result.element;
     if (element == null) throw ArgumentError.notNull('element');
 
-    // Initialize [packageGraph]'s cache of ModelNodes for relevant
+    // Initialize [packageGraph]'s cache of [ModelNode]s for relevant
     // elements in this library.
-    var _compilationUnitMap = <String, CompilationUnit>{};
-    _compilationUnitMap.addEntries(resolvedLibrary.result.units
-        .map((ResolvedUnitResult u) => MapEntry(u.path, u.unit)));
+    var compilationUnitMap = <String, CompilationUnit>{
+      for (var unit in resolvedLibrary.result.units) unit.path: unit.unit,
+    };
     _HashableChildLibraryElementVisitor((Element e) =>
-            packageGraph.populateModelNodeFor(e, _compilationUnitMap))
+            packageGraph.populateModelNodeFor(e, compilationUnitMap))
         .visitElement(element);
 
     var exportedAndLocalElements = {

test/end2end/dartdoc_integration_test.dart

@@ -227,13 +227,13 @@ void main() {
     }, timeout: Timeout.factor(2));
 
     test('--footer-text excludes version', () async {
-      var _testPackagePath = path.fromUri(
+      var testPackagePath = path.fromUri(
           _currentFileUri.resolve('../../testing/test_package_options'));
 
       var args = <String>[dartdocPath, '--output', tempDir.path];
 
       await subprocessLauncher.runStreamed(Platform.resolvedExecutable, args,
-          workingDirectory: _testPackagePath);
+          workingDirectory: testPackagePath);
 
       var outFile = File(path.join(tempDir.path, 'index.html'));
       var footerRegex =