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

lib/compiler/implementation/resolver.dart

Index: lib/compiler/implementation/resolver.dart
diff --git a/lib/compiler/implementation/resolver.dart b/lib/compiler/implementation/resolver.dart
index 06f4b3fcc148b0531c57660a4d57940a20fb1ad7..7c655482aed2518943bf5b071d79c800f6c43a18 100644
--- a/lib/compiler/implementation/resolver.dart
+++ b/lib/compiler/implementation/resolver.dart
@@ -89,7 +89,9 @@ class ResolverTask extends CompilerTask {
       final SourceString constructorName =
           getConstructorName(initializers.head);
       final SourceString className = classElement.name;
-      return classElement.lookupConstructor(className, constructorName);
+      return classElement.lookupConstructor(constructor.getLibrary(),
+                                            className,
+                                            constructorName);
     }
     return null;
   }
@@ -185,7 +187,9 @@ class ResolverTask extends CompilerTask {
     } else {
       name = constructor.name;
     }
-    constructor.defaultImplementation = defaultClass.lookupConstructor(name);
+    constructor.defaultImplementation =
+        defaultClass.lookupConstructor(defaultClass.getLibrary(),
+                                       name);
 
     if (constructor.defaultImplementation === null) {
       // We failed to find a constructor named either
@@ -592,7 +596,10 @@ class InitializerResolver {
     // Lookup constructor and try to match it to the selector.
     ResolverTask resolver = visitor.compiler.resolver;
     final SourceString className = lookupTarget.name;
-    result = lookupTarget.lookupConstructor(className, constructorName);
+    result =
+      lookupTarget.lookupConstructor(constructor.getLibrary(),
+                                     className,
+                                     constructorName);
     if (result === null || !result.isGenerativeConstructor()) {
       String classNameString = className.slowToString();
       String constructorNameString = constructorName.slowToString();
@@ -2596,7 +2603,10 @@ class ConstructorResolver extends CommonResolverVisitor<Element> {
                                     Node diagnosticNode,
                                     SourceString constructorName) {
     cls.ensureResolved(compiler);
-    Element result = cls.lookupConstructor(cls.name, constructorName);
+    Element result =
+        cls.lookupConstructor(resolver.enclosingElement.getLibrary(),
+                              cls.name,
+                              constructorName);
     if (result === null) {
       String fullConstructorName = cls.name.slowToString();
       if (constructorName !== const SourceString('')) {