fix reference marking in for-in loops (#1535)

fixes #1533

lib/compress.js

@@ -244,7 +244,11 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_ForIn) {
                     if (node.init instanceof AST_SymbolRef) {
-                        node.init.definition().fixed = false;
+                        var d = node.init.definition();
+                        d.references.push(node.init);
+                        d.fixed = false;
+                    } else {
+                        node.init.walk(tw);
                     }
                     node.object.walk(tw);
                     push();

test/compress/reduce_vars.js

@@ -589,3 +589,39 @@ inner_var_for_in: {
         x(1, b, c, d);
     }
 }
+
+issue_1533_1: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+    }
+    input: {
+        var id = "";
+        for (id in {break: "me"})
+            console.log(id);
+    }
+    expect: {
+        var id = "";
+        for (id in {break: "me"})
+            console.log(id);
+    }
+}
+
+issue_1533_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        var id = "";
+        for (var id in {break: "me"})
+            console.log(id);
+        console.log(id);
+    }
+    expect: {
+        var id = "";
+        for (var id in {break: "me"})
+            console.log(id);
+        console.log(id);
+    }
+}