refactor(compiler): Remove NullAstVisitor and visitAstChildren (#35619)

This commit removes the `NullAstVisitor` and `visitAstChildren` exported
from `packages/compiler/src/expression_parser/ast.ts` because they
contain duplicate and buggy implementation, and their use cases could be
sufficiently covered by `RecursiveAstVisitor` if the latter implements the
`visit` method. This use case is only needed in the language service.

With this change, any visitor that extends `RecursiveAstVisitor` could
just define their own `visit` function and the parent class will behave
correctly.

A bit of historical context:
In language service, we need a way to tranverse the expression AST in a
selective manner based on where the user's cursor is. This means we need a
"filtering" function to decide which node to visit and which node to not
visit. Instead of refactoring `RecursiveAstVisitor` to support this,
`visitAstChildren` was created. `visitAstChildren` duplicates the
implementation of `RecursiveAstVisitor`, but introduced some bugs along
the way. For example, in `visitKeyedWrite`, it visits
```
obj -> key -> obj
```
instead of
```
obj -> key -> value
```

Moreover, because of the following line
```
visitor.visit && visitor.visit(ast, context) || ast.visit(visitor, context);
```
`visitAstChildren` visits every node *twice*.

PR Close #35619

packages/language-service/src/expressions.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AST, ASTWithSource, AstPath as AstPathBase, NullAstVisitor, visitAstChildren} from '@angular/compiler';
+import {AST, ASTWithSource, AstPath as AstPathBase, RecursiveAstVisitor} from '@angular/compiler';
 import {AstType} from './expression_type';
 
 import {BuiltinType, Span, Symbol, SymbolQuery, SymbolTable} from './types';
@@ -16,12 +16,12 @@ type AstPath = AstPathBase<AST>;
 
 function findAstAt(ast: AST, position: number, excludeEmpty: boolean = false): AstPath {
   const path: AST[] = [];
-  const visitor = new class extends NullAstVisitor {
+  const visitor = new class extends RecursiveAstVisitor {
     visit(ast: AST) {
       if ((!excludeEmpty || ast.sourceSpan.start < ast.sourceSpan.end) &&
           inSpan(position, ast.sourceSpan)) {
         path.push(ast);
-        visitAstChildren(ast, this);
+        ast.visit(this);
       }
     }
   };