Made dart2js constructor lookup logic "private"-aware, fixed 4740 bug.

lib/compiler/implementation/dart_backend/placeholder_collector.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.
 
 class LocalPlaceholder implements Hashable {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 428 matching lines @@
       if (node.typeName is Send) {
         final element = treeElements[node];
         if (element !== null) {
           final send = node.typeName.asSend();
           Identifier receiver = send.receiver;
           Identifier selector = send.selector;
           hasPrefix() {
             if (element is TypedefElement) return true;
             ClassElement classElement = element;
             final constructor = classElement.lookupConstructor(
-                receiver.source, selector.source);
+                currentElement.getLibrary(),
+                receiver.source,
+                selector.source);
             return constructor === null;
           }
           if (!hasPrefix()) target = send.receiver;
         }
       }
       // TODO(antonm): is there a better way to detect unresolved types?
       if (type.element !== compiler.types.dynamicType.element) {
         makeTypePlaceholder(target, type);
       } else {
         if (!isDynamicType(node)) makeUnresolvedPlaceholder(target);
@@ skipping 124 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }