Fix analyzer warnings in assertions of the TypeExpression CL

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

 // Copyright (c) 2013, 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 dart2js.ir_nodes;
 
 import 'dart:collection';
 import 'cps_fragment.dart' show CpsFragment;
 import 'cps_ir_nodes_sexpr.dart';
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
@@ skipping 1847 matching lines @@
 class TypeExpression extends Primitive {
   final TypeExpressionKind kind;
   final DartType dartType;
   final List<Reference<Primitive>> arguments;
 
   TypeExpression(this.kind,
                  this.dartType,
                  List<Primitive> arguments)
       : this.arguments = _referenceList(arguments) {
     assert(kind == TypeExpressionKind.INSTANCE
-           ? dartType == dartType.element.thisType
+           ? dartType == (dartType.element as ClassElement).thisType
            : true);
   }
 
   @override
   accept(Visitor visitor) {
     return visitor.visitTypeExpression(this);
   }
 
   bool get hasValue => true;
   bool get isSafeForElimination => true;
@@ skipping 1027 matching lines @@
     plug(new Branch.loose(_definitions.getCopy(node.condition),
         _copies[node.trueContinuation.definition],
         _copies[node.falseContinuation.definition])
       ..isStrictCheck = node.isStrictCheck);
   }
 
   visitUnreachable(Unreachable node) {
     plug(new Unreachable());
   }
 }