Fix invariant in ResolvedVisitor.

pkg/compiler/lib/src/resolved_visitor.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.
 
 part of dart2js;
 
 /// Enum for the visit methods added in [ResolvedVisitor].
 // TODO(johnniwinther): Remove this.
 enum ResolvedKind {
   ASSERT,
@@ skipping 98 matching lines @@
       if (!Elements.isUnresolved(element) && element.impliesType) {
         return visitor.visitTypePrefixSend(node);
       } else {
         return visitor.visitGetterSend(node);
       }
     } else if (element != null && Initializers.isConstructorRedirect(node)) {
       return visitor.visitStaticSend(node);
     } else if (Elements.isClosureSend(node, element)) {
       return visitor.visitClosureSend(node);
     } else {
-      if (Elements.isUnresolved(element)) {
+      if (node.isConditional) {
+        return visitor.visitDynamicSend(node);
+      } else if (Elements.isUnresolved(element)) {
         if (element == null) {
           // Example: f() with 'f' unbound.
           // This can only happen inside an instance method.
           return visitor.visitDynamicSend(node);
         } else {
           return visitor.visitStaticSend(node);
         }
       } else if (element.isInstanceMember) {
         // Example: f() with 'f' bound to instance method.
         return visitor.visitDynamicSend(node);
@@ skipping 658 matching lines @@
 
   @override
   R visitSuperIndex(
       Send node,
       FunctionElement function,
       Node index,
       ResolvedKindVisitor<R> visitor) {
     return visitor.visitSuperSend(node);
   }
 }