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

tests/compiler/dart2js/mock_compiler.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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 159 matching lines @@
   print(var obj) {}
   abstract class num {}
   abstract class int extends num { }
   abstract class double extends num {
     static var NAN = 0;
     static parse(s) {}
   }
   class bool {}
   class String implements Pattern {}
   class Object {
+    Object();
     operator ==(other) { return true; }
     get hashCode => throw "Object.hashCode not implemented.";
     String toString() { return null; }
     noSuchMethod(im) { throw im; }
   }
   abstract class StackTrace {}
   class Type {}
   class Function {}
   class List<E> {
     List([length]);
@@ skipping 261 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }