Change signature of lookupConstructor to only require a name.

pkg/compiler/lib/src/elements/elements.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library elements;
 
 
 import '../constants/expressions.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 1360 matching lines @@
   Element lookupBackendMember(String memberName);
   Element lookupSuperMember(String memberName);
 
   Element lookupSuperMemberInLibrary(String memberName,
                                      LibraryElement library);
 
   Element validateConstructorLookupResults(Selector selector,
                                            Element result,
                                            Element noMatch(Element));
 
-  Element lookupConstructor(Selector selector, [Element noMatch(Element)]);
+  ConstructorElement lookupDefaultConstructor();
+  ConstructorElement lookupConstructor(String name);
 
   void forEachMember(void f(ClassElement enclosingClass, Element member),
                      {bool includeBackendMembers: false,
                       bool includeSuperAndInjectedMembers: false});
 
   void forEachInstanceField(void f(ClassElement enclosingClass,
                                    FieldElement field),
                             {bool includeSuperAndInjectedMembers: false});
 
   /// Similar to [forEachInstanceField] but visits static fields.
@@ skipping 221 matching lines @@
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }