fix break in r3504

frog/value.dart

 // Copyright (c) 2012, 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.
 
 
 interface CallingContext {
   MemberSet findMembers(String name);
   CounterLog get counters();
   Library get library();
   bool get isStatic();
@@ skipping 26 matching lines @@
   // TODO(jimhug): Can we remove span?
   PureStaticValue(Type type, SourceSpan span,
     [this.isConst = false, this.isType = false]):
     super(type, null, span);
 
   Member getMem(CallingContext context, String name, Node node) {
     var member = type.getMember(name);
 
     if (member == null) {
       world.warning('can not find "$name" on "${type.name}"', node.span);
+      return null;
     }
 
     if (isType && !member.isStatic) {
       world.error('can not refer to instance member as static', node.span);
     }
 
     return member;
   }
 
   Value get_(CallingContext context, String name, Node node) {
@@ skipping 1554 matching lines @@
     }
     return super.unop(kind, context, node);
   }
   Value binop(int kind, var other, CallingContext context, var node) {
     if (value != null) {
       return replaceValue(value.binop(kind, _unwrap(other), context, node));
     }
     return super.binop(kind, other, context, node);
   }
 }