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

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

Index: sdk/lib/_internal/compiler/implementation/closure.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/closure.dart	(revision 25147)
+++ sdk/lib/_internal/compiler/implementation/closure.dart	(working copy)
@@ -42,6 +42,8 @@
       // cache. One for given node and one for each nested closure.
       if (node is FunctionExpression) {
         translator.translateFunction(element, node);
+      } else if (element.isSynthesized) {
+        return new ClosureClassMap(null, null, null, new ThisElement(element));
       } else {
         // Must be the lazy initializer of a static.
         assert(node is SendSet);