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 72e3a9abfe799df7f460665595cecc8af2774902..cbcb6d2604037815de6311541801c19e6144e60c 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -1275,7 +1275,8 @@ class ClassElement extends ScopeContainerElement
     }
   }
 
-  Element lookupConstructor(SourceString className,
+  Element lookupConstructor(LibraryElement fromLibrary,

[kasperl, 2012/09/19 05:59:17]

In general, I think we should start using selectors as keys for these kind of
lookups.

+                            SourceString className,
                             [SourceString constructorName =
                                  const SourceString(''),
                             Element noMatch(Element)]) {
@@ -1289,7 +1290,10 @@ class ClassElement extends ScopeContainerElement
       normalizedName = className;
     }
     Element result = localLookup(normalizedName);
-    if (result === null || !result.isConstructor()) {
+    if (result === null
+        || !result.isConstructor()
+        || (constructorName.isPrivate()
+            && result.getLibrary() != fromLibrary)) {
       result = noMatch !== null ? noMatch(result) : null;
     }
     return result;