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..b7b7f1c865c844e4b010415e3cb4464d9fa715a9 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -1275,10 +1275,11 @@ class ClassElement extends ScopeContainerElement
     }
   }
 
-  Element lookupConstructor(SourceString className,
-                            [SourceString constructorName =
-                                 const SourceString(''),
-                            Element noMatch(Element)]) {
+  Element lookupConstructor(Selector selector, [Element noMatch(Element)]) {
+    SourceString className = selector.selectorName.className;

[kasperl, 2012/09/21 12:12:06]

This makes it look like all you really need is to have the name you stuff in the
selector be pre-normalized somehow so that it contains everything you need to do
the lookup?

I understand there may be some complications in figuring out a selector is
private or not (based on the name alone), but maybe we could change
Selector.isPrivate to say yes if Selector.library != null and let the name in a
selector for a private named constructor be something like 'C._internal'?

[aam-me, 2012/09/21 12:35:27]

Right.
Right, we can do that. My thoughts we that storing name normalized (lib name,
class name, constructor name) is better than parsing name every time we do
lookup. But then again, parsing here is just splitting by '.', so it is not that
expensive. 

+    SourceString constructorName = selector.selectorName.name;
+
+    LibraryElement fromLibrary = selector.library;
     // TODO(karlklose): have a map from class names to a map of constructors
     //                  instead of creating the name here?
     SourceString normalizedName;
@@ -1289,7 +1290,10 @@ class ClassElement extends ScopeContainerElement
       normalizedName = className;
     }
     Element result = localLookup(normalizedName);
-    if (result === null || !result.isConstructor()) {
+    if (result === null
+        || !result.isConstructor()
+        || (selector.selectorName.isPrivate()
+            && result.getLibrary() != fromLibrary)) {
       result = noMatch !== null ? noMatch(result) : null;
     }
     return result;