[frontend_server] Move ToStringTransformer to the Dart repo

The reduces the dependency of flutter/engine on the cfe internals.

Context: https://dart-review.googlesource.com/c/sdk/+/172180 had to be
reverted because the refactoring of the package:kernel code broke
the flutter/engine.

A follow-up CL is needed in the flutter repo to use the moved classes
instead.

Change-Id: I674ab3a09569d615b1549a5465da0f658945ec7e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/172762
Reviewed-by: Dan Field <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/frontend_server/lib/src/to_string_transformer.dart

@@ -0,0 +1,94 @@
+// 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 'package:kernel/visitor.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<void> {
+  /// 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);
+  }
+}