fix analyzer error

pkg/polymer_expressions/lib/eval.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 polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 
 @MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
@@ skipping 477 matching lines @@
   _updateSelf(Scope scope) {
     var f = _BINARY_OPERATORS[operator];
     if (operator == '&&' || operator == '||') {
       _value = f(_toBool(left._value), _toBool(right._value));
     } else if (operator == '==' || operator == '!=') {
       _value = f(left._value, right._value);
     } else if (left._value == null || right._value == null) {
       _value = null;
     } else {
       if (operator == '|' && left._value is ObservableList) {
-        _subscription = left._value.listChanges.listen(
-            (_) => _invalidate(scope));
+        _subscription = (left._value as ObservableList).listChanges
+            .listen((_) => _invalidate(scope));
       }
       _value = f(left._value, right._value);
     }
   }
 
   accept(Visitor v) => v.visitBinaryOperator(this);
 
 }
 
 class InvokeObserver extends ExpressionObserver<Invoke> implements Invoke {
@@ skipping 134 matching lines @@
    * This does not work for calls that need to pass more than one argument.
    */
   call(arg0) => mirror.invoke(symbol, [arg0], null).reflectee;
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }