fix #25578, implement @covariant parameter overrides

pkg/analyzer/lib/src/dart/element/element.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2440 matching lines @@
 
   @override
   accept(ElementVisitor visitor) => null;
 }
 
 /**
  * A concrete implementation of an [ElementAnnotation].
  */
 class ElementAnnotationImpl implements ElementAnnotation {
   /**
+   * The name of the top-level variable used to mark a method as covariant.
+   */
+  static String _COVARIANT_VARIABLE_NAME = "covariant";
+
+  /**
    * The name of the class used to mark an element as being deprecated.
    */
   static String _DEPRECATED_CLASS_NAME = "Deprecated";
 
   /**
    * The name of the top-level variable used to mark an element as being
    * deprecated.
    */
   static String _DEPRECATED_VARIABLE_NAME = "deprecated";
 
@@ skipping 82 matching lines @@
    * compilation unit in which the annotation appears.
    */
   ElementAnnotationImpl(this.compilationUnit);
 
   @override
   DartObject get constantValue => evaluationResult?.value;
 
   @override
   AnalysisContext get context => compilationUnit.library.context;
 
+  /**
+   * Return `true` if this annotation marks the associated parameter as being
+   * covariant, meaning it is allowed to have a narrower type in an override.
+   */
+  bool get isCovariant =>
+      element is PropertyAccessorElement &&
+      element.name == _COVARIANT_VARIABLE_NAME &&
+      element.library?.name == _META_LIB_NAME;
+
   @override
   bool get isDeprecated {
     if (element?.library?.isDartCore == true) {
       if (element is ConstructorElement) {
         return element.enclosingElement.name == _DEPRECATED_CLASS_NAME;
       } else if (element is PropertyAccessorElement) {
         return element.name == _DEPRECATED_VARIABLE_NAME;
       }
     }
     return false;
@@ skipping 4361 matching lines @@
    */
   int _visibleRangeOffset = 0;
 
   /**
    * The length of the visible range for this element, or `-1` if this element
    * does not have a visible range.
    */
   int _visibleRangeLength = -1;
 
   /**
+   * True if this parameter inherits `@covariant` because it overrides a
+   * method in a supertype that has this parameter as covariant.
+   */
+  bool inheritsCovariant = false;
+
+  /**
    * Initialize a newly created parameter element to have the given [name] and
    * [nameOffset].
    */
   ParameterElementImpl(String name, int nameOffset)
       : _unlinkedParam = null,
         super(name, nameOffset);
 
   /**
    * Initialize a newly created parameter element to have the given [name].
    */
@@ skipping 134 matching lines @@
 
   @override
   bool get isConst {
     if (_unlinkedParam != null) {
       return false;
     }
     return super.isConst;
   }
 
   @override
+  bool get isCovariant {
+    if (inheritsCovariant) {
+      return true;
+    }
+    for (ElementAnnotationImpl annotation in metadata) {
+      if (annotation.isCovariant) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  @override
   bool get isFinal {
     if (_unlinkedParam != null) {
       return false;
     }
     return super.isFinal;
   }
 
   @override
   bool get isInitializingFormal => false;
 
@@ skipping 1396 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }