Fix #28120, strong mode allows field overrides

pkg/dev_compiler/lib/src/compiler/element_helpers.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.
 
 import 'dart:collection';
 
 /// Helpers for Analyzer's Element model and corelib model.
 
 import 'package:analyzer/dart/ast/ast.dart'
     show
         ConstructorDeclaration,
         Expression,
         FunctionBody,
         FunctionExpression,
         MethodDeclaration,
         MethodInvocation,
         SimpleIdentifier;
 import 'package:analyzer/dart/element/element.dart'
-    show ClassElement, Element, ExecutableElement, FunctionElement;
+    show
+        ClassElement,
+        Element,
+        ExecutableElement,
+        FunctionElement,
+        LibraryElement;
 import 'package:analyzer/dart/element/type.dart'
     show DartType, InterfaceType, ParameterizedType;
 import 'package:analyzer/src/dart/element/type.dart' show DynamicTypeImpl;
 import 'package:analyzer/src/generated/constant.dart'
     show DartObject, DartObjectImpl;
 
 class Tuple2<T0, T1> {
   final T0 e0;
   final T1 e1;
   Tuple2(this.e0, this.e1);
@@ skipping 102 matching lines @@
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }
+
+List<ClassElement> getImmediateSuperclasses(ClassElement c) {
+  var result = <ClassElement>[];
+  for (var m in c.mixins.reversed) {
+    result.add(m.element);
+  }
+  var s = c.supertype;
+  if (s != null) result.add(s.element);
+  return result;
+}
+
+bool isSdkInternalRuntime(LibraryElement l) =>
+    l.isInSdk && l.source.uri.toString() == 'dart:_runtime';