Revert of VariableProxy: when cloning, don't even think about creating dangling references.

src/ast/ast.cc

Index: src/ast/ast.cc
diff --git a/src/ast/ast.cc b/src/ast/ast.cc
index 4ecdcf1bb44b81e3ab6a6179de0323466e5842a3..c4808ca840500bd89c9c4b1498d2f5f16733ad2e 100644
--- a/src/ast/ast.cc
+++ b/src/ast/ast.cc
@@ -186,8 +186,11 @@
       end_position_(copy_from->end_position_),
       next_unresolved_(nullptr) {
   bit_field_ = copy_from->bit_field_;
-  DCHECK(!copy_from->is_resolved());
-  raw_name_ = copy_from->raw_name_;
+  if (copy_from->is_resolved()) {
+    var_ = copy_from->var_;
+  } else {
+    raw_name_ = copy_from->raw_name_;
+  }
 }
 
 void VariableProxy::BindTo(Variable* var) {