Rewrite how we handle synthesized constructors in the compiler. This was motivated by issue https:/…

sdk/lib/_internal/compiler/implementation/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 'modelx.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 827 matching lines @@
   bool isObject(Compiler compiler);
   bool isSubclassOf(ClassElement cls);
   bool implementsInterface(ClassElement intrface);
   bool isShadowedByField(Element fieldMember);
 
   ClassElement ensureResolved(Compiler compiler);
 
   void addMember(Element element, DiagnosticListener listener);
   void addToScope(Element element, DiagnosticListener listener);
 
-  /**
-   * Add a synthetic nullary constructor if there are no other
-   * constructors.
-   */
-  void addDefaultConstructorIfNeeded(Compiler compiler);
+  void setDefaultConstructor(FunctionElement constructor, Compiler compiler);
 
   void addBackendMember(Element element);
   void reverseBackendMembers();
 
   Element lookupMember(SourceString memberName);
   Element lookupSelector(Selector selector, Compiler compiler);
   Element lookupSuperSelector(Selector selector, Compiler compiler);
 
   Element lookupLocalMember(SourceString memberName);
   Element lookupBackendMember(SourceString memberName);
@@ skipping 77 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }