Revert "Change signature of lookupConstructor to only require a name."

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 0d66731ff0363e39bf4550354a8f5b952616b75c..969da6f98a40c5b03ee9977f297a73cea5c6af49 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -842,16 +842,15 @@ class JavaScriptBackend extends Backend {
           // The constructor is on the patch class, but dart2js unit tests don't
           // have a patch class.
           ClassElement implementation = cls.patch != null ? cls.patch : cls;
-          ConstructorElement ctor = implementation.lookupConstructor(name);
-          if (ctor == null
-              || (isPrivateName(name)
-                  && ctor.library != mapLiteralClass.library)) {
-            compiler.internalError(mapLiteralClass,
-                                   "Map literal class $mapLiteralClass missing "
-                                   "'$name' constructor"
-                                   "  ${mapLiteralClass.constructors}");
-          }
-          return ctor;
+          return implementation.lookupConstructor(
+            new Selector.callConstructor(
+                name, mapLiteralClass.library, arity),
+            (element) {
+              compiler.internalError(mapLiteralClass,
+                  "Map literal class $mapLiteralClass missing "
+                  "'$name' constructor"
+                  "  ${mapLiteralClass.constructors}");
+            });
         }
         mapLiteralConstructor = getFactory('_literal', 1);
         mapLiteralConstructorEmpty = getFactory('_empty', 0);