Check in codegen test expectations.

test/codegen/expect/language/issue12288_test.js

Index: test/codegen/expect/language/issue12288_test.js
diff --git a/test/codegen/expect/language/issue12288_test.js b/test/codegen/expect/language/issue12288_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..2789ccd9b950844b36c6f804b350b6c438663066
--- /dev/null
+++ b/test/codegen/expect/language/issue12288_test.js
@@ -0,0 +1,40 @@
+dart_library.library('language/issue12288_test', null, /* Imports */[
+  'dart_sdk'
+], function load__issue12288_test(exports, dart_sdk) {
+  'use strict';
+  const core = dart_sdk.core;
+  const _interceptors = dart_sdk._interceptors;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const issue12288_test = Object.create(null);
+  let JSArrayOfElement = () => (JSArrayOfElement = dart.constFn(_interceptors.JSArray$(issue12288_test.Element)))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  issue12288_test.main = function() {
+    let parent = new issue12288_test.Element(null);
+    let child = new issue12288_test.Element(parent);
+    let result = child.path0[dartx.length];
+    if (result != 2) {
+      dart.throw(dart.str`Expected 2, but child.path0.length was ${result}`);
+    }
+  };
+  dart.fn(issue12288_test.main, VoidTodynamic());
+  issue12288_test.Element = class Element extends core.Object {
+    new(parent) {
+      this.parent = parent;
+    }
+    get path0() {
+      if (this.parent == null) {
+        return JSArrayOfElement().of([this]);
+      } else {
+        let list = this.parent.path0;
+        list[dartx.add](this);
+        return list;
+      }
+    }
+  };
+  dart.setSignature(issue12288_test.Element, {
+    constructors: () => ({new: dart.definiteFunctionType(issue12288_test.Element, [issue12288_test.Element])})
+  });
+  // Exports:
+  exports.issue12288_test = issue12288_test;
+});