Issue 22962. Adding/removing method/accessor body is a declartion mismatch.

pkg/analyzer/lib/src/generated/incremental_resolver.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 engine.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/services/lint.dart';
@@ skipping 250 matching lines @@
     } else {
       element = _findElement(_enclosingUnit.accessors, name);
     }
     // process element
     _processElement(element);
     _assertSameAnnotations(node, element);
     _assertFalse(element.isSynthetic);
     _assertSameType(node.returnType, element.returnType);
     _assertCompatibleParameters(
         node.functionExpression.parameters, element.parameters);
-    _assertBodyModifiers(node.functionExpression.body, element);
+    _assertBody(node.functionExpression.body, element);
     // matches, update the existing element
     ExecutableElement newElement = node.element;
     node.name.staticElement = element;
     node.functionExpression.element = element;
     _setLocalElements(element, newElement);
   }
 
   @override
   visitFunctionTypeAlias(FunctionTypeAlias node) {
     String name = node.name.name;
@@ skipping 53 matching lines @@
       element = _findElement(_enclosingClass.accessors, name);
     }
     // process element
     ExecutableElement newElement = node.element;
     try {
       _assertNotNull(element);
       _assertSameAnnotations(node, element);
       _assertEquals(node.isStatic, element.isStatic);
       _assertSameType(node.returnType, element.returnType);
       _assertCompatibleParameters(node.parameters, element.parameters);
-      _assertBodyModifiers(node.body, element);
+      _assertBody(node.body, element);
       _removedElements.remove(element);
       // matches, update the existing element
       node.name.staticElement = element;
       _setLocalElements(element, newElement);
     } on _DeclarationMismatchException {
       _addedElements.add(newElement);
       _removeElement(element);
       // add new element
       if (newElement is MethodElement) {
         List<MethodElement> methods = _enclosingClass.methods;
@@ skipping 57 matching lines @@
   }
 
   @override
   visitWithClause(WithClause node) {
     List<TypeName> nodes = node.mixinTypes;
     List<InterfaceType> types = _enclosingClass.mixins;
     _assertSameTypes(nodes, types);
   }
 
   /**
-   * Asserts that [body] has async / generator modifiers compatible with the
-   * given [element].
+   * Assert that the given [body] is compatible with the given [element].
+   * It should not be empty if the [element] is not an abstract class member.
+   * If it is present, it should have the same async / generator modifiers.
    */
-  void _assertBodyModifiers(FunctionBody body, ExecutableElementImpl element) {
-    _assertEquals(body.isSynchronous, element.isSynchronous);
-    _assertEquals(body.isGenerator, element.isGenerator);
+  void _assertBody(FunctionBody body, ExecutableElementImpl element) {
+    if (body is EmptyFunctionBody) {
+      _assertTrue(element.isAbstract);
+    } else {
+      _assertFalse(element.isAbstract);
+      _assertEquals(body.isSynchronous, element.isSynchronous);
+      _assertEquals(body.isGenerator, element.isGenerator);
+    }
   }
 
   void _assertCombinators(List<Combinator> nodeCombinators,
       List<NamespaceCombinator> elementCombinators) {
     // prepare shown/hidden names in the element
     Set<String> showNames = new Set<String>();
     Set<String> hideNames = new Set<String>();
     for (NamespaceCombinator combinator in elementCombinators) {
       if (combinator is ShowElementCombinator) {
         showNames.addAll(combinator.shownNames);
@@ skipping 1371 matching lines @@
   @override
   String toString() => name;
 }
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }