Don't remove trailing spaces in WritingLints.MD

doc/WritingLints.MD

@@ -105,6 +105,10 @@ You'll notice when authoring a new rule that failures cause the AST of the test
 
 would dump the AST of `rules.dart`.
 
+### Performance
+
+For performance reasons rules should prefer implementing `NodeLintRule` and registering interest in specific AST node types using `registry.addXYZ(this, visitor)`. Avoid overriding `visitCompilationUnit()` and performing your own full `CompilationUnit` visits.
+
 # Feedback is Welcome!
 
 Details are under active development.  Feedback is most [welcome](https://github.com/dart-lang/linter/issues)!

tool/rule.dart

@@ -122,22 +122,29 @@ const _details = r'''
 
 ''';
 
-class $className extends LintRule {
+class $className extends LintRule implements NodeLintRule {
   $className() : super(
           name: '$ruleName',
             description: _desc,
             details: _details,
             group: Group.style);
 
   @override
-  AstVisitor getVisitor() => new Visitor(this);
+  void registerNodeProcessors(NodeLintRegistry registry) {
+    final visitor = new _Visitor(this);
+    registry.addSimpleIdentifier(this, visitor);
+  }
 }
 
-class Visitor extends SimpleAstVisitor {
+class _Visitor extends SimpleAstVisitor {
   final LintRule rule;
-  Visitor(this.rule);
 
+  _Visitor(this.rule);
 
+  @override
+  void visitSimpleIdentifier(SimpleIdentifier node) {
+    // TODO: implement
+  }
 }
 """;