[vm/aot] Visit TypeParameterType.promotedBound in tree shaker

If tree shaker doesn't visit TypeParameterType.promotedBound which
contains the only reference to a particular class it may remove that
class, leaving a dangling reference from promotedBound. After that
kernel AST serialization would crash with "Missing canonical name for
Reference" error.

This change also fixes TypeParameterType.visitChildren.

TEST=pkg/vm/testcases/transformations/type_flow/transformer/regress_47878.dart
Fixes #47878

Change-Id: I555261713240bf0be412db27efb68f32cfd8008b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222781
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

pkg/kernel/lib/ast.dart

@@ -11888,7 +11888,9 @@ class TypeParameterType extends DartType {
       v.visitTypeParameterType(this, arg);
 
   @override
-  void visitChildren(Visitor v) {}
+  void visitChildren(Visitor v) {
+    promotedBound?.accept(v);
+  }
 
   @override
   bool operator ==(Object other) => equals(other, null);

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -985,6 +985,7 @@ class _TreeShakerTypeVisitor extends RecursiveVisitor {
     if (parent is Class) {
       shaker.addClassUsedInType(parent);
     }
+    node.visitChildren(this);
   }
 }
 

pkg/vm/testcases/transformations/type_flow/transformer/regress_47878.dart

@@ -0,0 +1,27 @@
+// Copyright (c) 2021, 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.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/47878
+
+abstract class Disposable {}
+
+class A {}
+
+class Data<T> {
+  T? value;
+}
+
+class DataStream<T> {
+  DataStream({newValue, Data<T>? stream}) {
+    var lastValue = stream!.value;
+
+    if (lastValue != null &&
+        lastValue is Disposable &&
+        lastValue != newValue) {}
+  }
+}
+
+void main() {
+  DataStream<A>();
+}

pkg/vm/testcases/transformations/type_flow/transformer/regress_47878.dart.expect

@@ -0,0 +1,26 @@
+library #lib /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class Disposable extends core::Object {
+}
+abstract class A extends core::Object {
+}
+abstract class Data<T extends core::Object? = dynamic> extends core::Object {
+}
+class DataStream<T extends core::Object? = dynamic> extends core::Object {
+  constructor •() → self::DataStream<self::DataStream::T%>
+    : super core::Object::•() {
+    self::DataStream::T? lastValue = block {
+      #C1!;
+    } =>throw "Attempt to execute code removed by Dart AOT compiler (TFA)";
+    if(!(throw "Attempt to execute code removed by Dart AOT compiler (TFA)") && false && !([@vm.inferred-type.metadata=dart.core::bool (skip check) (receiver not int)] lastValue{self::DataStream::T & self::Disposable /* '!' & '!' = '!' */} =={core::Object::==}{(core::Object) → core::bool} #C1)) {
+    }
+  }
+}
+static method main() → void {
+  new self::DataStream::•<self::A>();
+}
+constants  {
+  #C1 = null
+}