re-land fix #27110 with proper DDC side of changes

pkg/analyzer/lib/src/task/strong/ast_properties.dart

 // Copyright (c) 2016, 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.
 
 /// Properties that result from Strong Mode analysis on an AST.
 ///
 /// These properties are not public, but provided by use of back-ends such as
 /// Dart Dev Compiler.
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/element/type.dart';
 
+const String _implicitAssignmentCast = '_implicitAssignmentCast';
 const String _implicitCast = '_implicitCast';
 const String _hasImplicitCasts = '_hasImplicitCasts';
 const String _isDynamicInvoke = '_isDynamicInvoke';
 
 /// True if this compilation unit has any implicit casts, otherwise false.
 ///
 /// See also [getImplicitCast].
 bool hasImplicitCasts(CompilationUnit node) {
   return node.getProperty/*<bool>*/(_hasImplicitCasts) ?? false;
 }
 
 /// Sets [hasImplicitCasts] property for this compilation unit.
 void setHasImplicitCasts(CompilationUnit node, bool value) {
   node.setProperty(_hasImplicitCasts, value == true ? true : null);
 }
 
 /// If this expression has an implicit cast, returns the type it is coerced to,
 /// otherwise returns null.
 DartType getImplicitCast(Expression node) {
   return node.getProperty/*<DartType>*/(_implicitCast);
 }
 
 /// Sets the result of [getImplicitCast] for this node.
 void setImplicitCast(Expression node, DartType type) {
   node.setProperty(_implicitCast, type);
 }
 
+/// If this op-assign has an implicit cast on the assignment, returns the type
+/// it is coerced to, otherwise returns null.
+DartType getImplicitAssignmentCast(Expression node) {
+  return node.getProperty/*<DartType>*/(_implicitAssignmentCast);
+}
+
+/// Sets the result of [getImplicitAssignmentCast] for this node.
+void setImplicitAssignmentCast(Expression node, DartType type) {
+  node.setProperty(_implicitAssignmentCast, type);
+}
+
 /// True if this node is a dynamic operation that requires dispatch and/or
 /// checking at runtime.
 bool isDynamicInvoke(Expression node) {
   return node.getProperty/*<bool>*/(_isDynamicInvoke) ?? false;
 }
 
 /// Sets [isDynamicInvoke] property for this expression
 void setIsDynamicInvoke(Expression node, bool value) {
   node.setProperty(_isDynamicInvoke, value == true ? true : null);
 }