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

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

Index: sdk/lib/_internal/compiler/implementation/enqueue.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/enqueue.dart	(revision 25147)
+++ sdk/lib/_internal/compiler/implementation/enqueue.dart	(working copy)
@@ -105,12 +105,6 @@
   void addToWorkList(Element element) {
     assert(invariant(element, element.isDeclaration));
     if (element.isForeign(compiler)) return;
-
-    if (element.isForwardingConstructor) {
-      addToWorkList(element.targetConstructor);
-      return;
-    }
-
     internalAddToWorkList(element);
   }