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

sdk/lib/_internal/compiler/implementation/deferred_load.dart

 // Copyright (c) 2013, 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 deferred_load;
 
 import 'dart:collection'
        show LinkedHashMap,
             LinkedHashSet;
 
@@ skipping 222 matching lines @@
     Element dependency = elements[node];
     if (Elements.isUnresolved(dependency)) return;
     dependencies.add(dependency.implementation);
   }
 
   static Set<Element> collect(Element element, Compiler compiler) {
     TreeElements elements =
         compiler.enqueuer.resolution.getCachedElements(element);
     if (elements == null) return new LinkedHashSet<Element>();
     Node node = element.parseNode(compiler);
+    if (node == null) return new LinkedHashSet<Element>();
     var collector = new DependencyCollector(elements, compiler);
     node.accept(collector);
     collector.dependencies.addAll(elements.otherDependencies);
     return collector.dependencies;
   }
 }