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