Remove more type casts

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 analyzer.src.generated.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 405 matching lines @@
     _assertSameAnnotations(node, element);
     _assertEquals(node.isConst, element.isConst);
     _assertEquals(node.isFinal, element.isFinal);
     if (_enclosingFieldNode != null) {
       _assertEquals(_enclosingFieldNode.isStatic, element.isStatic);
     }
     _assertSameType(
         (node.parent as VariableDeclarationList).type, element.type);
     // matches, restore the existing element
     node.name.staticElement = element;
-    if (element is VariableElementImpl) {
-      (element as VariableElementImpl).initializer = newElement.initializer;
+    Element variable = element;
+    if (variable is VariableElementImpl) {
+      variable.initializer = newElement.initializer;
     }
   }
 
   @override
   visitWithClause(WithClause node) {
     List<TypeName> nodes = node.mixinTypes;
     List<InterfaceType> types = _enclosingClass.mixins;
     _assertSameTypes(nodes, types);
   }
 
@@ skipping 1798 matching lines @@
   @override
   String toString() => name;
 }
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }