fix corner case with import (#4672)

lib/scope.js

@@ -197,7 +197,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         } else if (node instanceof AST_SymbolLet) {
             scope.def_variable(node).exported = exported;
         } else if (node instanceof AST_SymbolVar) {
-            defun.def_variable(node, null).exported = exported;
+            defun.def_variable(node, node instanceof AST_SymbolImport ? undefined : null).exported = exported;
             entangle(defun, scope);
         }
 

test/compress/imports.js

@@ -128,3 +128,19 @@ rename_mangle: {
         import * as r from "moz";
     }
 }
+
+keep_ref: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        import foo from "bar";
+        foo();
+    }
+    expect: {
+        import foo from "bar";
+        foo();
+    }
+}