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

tests/utils/dummy_compiler_test.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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
 Future<String> provider(Uri uri) {
   String source;
   if (uri.scheme == "main") {
     source = "main() {}";
   } else if (uri.scheme == "lib") {
     if (uri.path.endsWith("/core.dart")) {
       source = """
 library core;
 class Object {
+  Object();
   operator==(other) {}
   get hashCode => throw 'Object.hashCode not implemented.';
 }
 class Type {}
 class bool {}
 class num {}
 class int {}
 class double{}
 class String{}
 class Function{}
@@ skipping 73 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   });
 }