Cleanup toString transformer

This is a final cleanup of old version of toString removal transformer
which was moved to pkg/vm in https://dart-review.googlesource.com/c/sdk/+/200525.

Also, this change removes handling of _KeepToString annotation class
after dart:ui @keepToString switched to use pragma and _KeepToString
was removed.

TEST=existing tests

Closes https://github.com/dart-lang/sdk/issues/46022

Change-Id: I7208ec70e6703d25b93d6ebbb306c18bae4b6987
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201162
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/frontend_server/lib/frontend_server.dart

@@ -38,8 +38,6 @@ import 'package:vm/kernel_front_end.dart';
 import 'src/javascript_bundle.dart';
 import 'src/strong_components.dart';
 
-export 'src/to_string_transformer.dart';
-
 ArgParser argParser = ArgParser(allowTrailingOptions: true)
   ..addFlag('train',
       help: 'Run through sample command line to produce snapshot',

pkg/frontend_server/lib/src/to_string_transformer.dart

@@ -1,93 +0,0 @@
-// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-import 'package:kernel/ast.dart';
-import '../frontend_server.dart';
-
-// Transformer/visitor for toString
-// If we add any more of these, they really should go into a separate library.
-
-/// A [RecursiveVisitor] that replaces [Object.toString] overrides with
-/// `super.toString()`.
-class ToStringVisitor extends RecursiveVisitor {
-  /// The [packageUris] must not be null.
-  ToStringVisitor(this._packageUris) : assert(_packageUris != null);
-
-  /// A set of package URIs to apply this transformer to, e.g. 'dart:ui' and
-  /// 'package:flutter/foundation.dart'.
-  final Set<String> _packageUris;
-
-  /// Turn 'dart:ui' into 'dart:ui', or
-  /// 'package:flutter/src/semantics_event.dart' into 'package:flutter'.
-  String _importUriToPackage(Uri importUri) =>
-      '${importUri.scheme}:${importUri.pathSegments.first}';
-
-  bool _isInTargetPackage(Procedure node) {
-    return _packageUris
-        .contains(_importUriToPackage(node.enclosingLibrary.importUri));
-  }
-
-  bool _hasKeepAnnotation(Procedure node) {
-    for (ConstantExpression expression
-        in node.annotations.whereType<ConstantExpression>()) {
-      if (expression.constant is! InstanceConstant) {
-        continue;
-      }
-      final InstanceConstant constant = expression.constant as InstanceConstant;
-      if (constant.classNode.name == '_KeepToString' &&
-          constant.classNode.enclosingLibrary.importUri.toString() ==
-              'dart:ui') {
-        return true;
-      }
-    }
-    return false;
-  }
-
-  @override
-  void visitProcedure(Procedure node) {
-    if (node.name.text == 'toString' &&
-        node.enclosingClass != null &&
-        node.enclosingLibrary != null &&
-        !node.isStatic &&
-        !node.isAbstract &&
-        !node.enclosingClass.isEnum &&
-        _isInTargetPackage(node) &&
-        !_hasKeepAnnotation(node)) {
-      node.function.body.replaceWith(
-        ReturnStatement(
-          SuperMethodInvocation(
-            node.name,
-            Arguments(<Expression>[]),
-          ),
-        ),
-      );
-    }
-  }
-
-  @override
-  void defaultMember(Member node) {}
-}
-
-/// Replaces [Object.toString] overrides with calls to super for the specified
-/// [packageUris].
-class ToStringTransformer extends ProgramTransformer {
-  /// The [packageUris] parameter must not be null, but may be empty.
-  ToStringTransformer(this._child, this._packageUris)
-      : assert(_packageUris != null);
-
-  final ProgramTransformer _child;
-
-  /// A set of package URIs to apply this transformer to, e.g. 'dart:ui' and
-  /// 'package:flutter/foundation.dart'.
-  final Set<String> _packageUris;
-
-  @override
-  void transform(Component component) {
-    assert(_child is! ToStringTransformer);
-    if (_packageUris.isNotEmpty) {
-      component.visitChildren(ToStringVisitor(_packageUris));
-    }
-    _child?.transform(component);
-  }
-}

pkg/vm/lib/transformations/to_string_transformer.dart

@@ -39,9 +39,6 @@ class ToStringVisitor extends RecursiveVisitor {
       final className = constant.classNode.name;
       final libraryUri =
           constant.classNode.enclosingLibrary.importUri.toString();
-      if (className == '_KeepToString' && libraryUri == 'dart:ui') {
-        return true;
-      }
       if (className == 'pragma' && libraryUri == 'dart:core') {
         for (var fieldRef in constant.fieldValues.keys) {
           if (fieldRef.asField.name.text == 'name') {