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

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index 6ecc0848ff4121fed3d06d52c5e05d18925784c2..a557fc3204e064dbc269b8161c3ca1e450ec0ebd 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -1376,21 +1376,14 @@ class ClassElement extends ScopeContainerElement
     }
   }
 
-  Element lookupConstructor(SourceString className,
-                            [SourceString constructorName =
-                                 const SourceString(''),
-                            Element noMatch(Element)]) {
+  Element lookupConstructor(Selector selector, [Element noMatch(Element)]) {
     // TODO(karlklose): have a map from class names to a map of constructors
     //                  instead of creating the name here?
-    SourceString normalizedName;
-    if (constructorName !== const SourceString('')) {
-      normalizedName = Elements.constructConstructorName(className,
-                                                         constructorName);
-    } else {
-      normalizedName = className;
-    }
-    Element result = localLookup(normalizedName);
-    if (result === null || !result.isConstructor()) {
+    Element result = localLookup(selector.normalizedConstructorName);
+    if (result === null
+        || !result.isConstructor()
+        || (selector.name.isPrivate()
+            && result.getLibrary() != selector.library)) {
       result = noMatch !== null ? noMatch(result) : null;
     }
     return result;