Check in codegen test expectations.

test/codegen/expect/language/regress_20840_test.js

Index: test/codegen/expect/language/regress_20840_test.js
diff --git a/test/codegen/expect/language/regress_20840_test.js b/test/codegen/expect/language/regress_20840_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..e242e8000892ad0c474fac8050f4b2965808413c
--- /dev/null
+++ b/test/codegen/expect/language/regress_20840_test.js
@@ -0,0 +1,35 @@
+dart_library.library('language/regress_20840_test', null, /* Imports */[
+  'dart_sdk'
+], function load__regress_20840_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 regress_20840_test = Object.create(null);
+  let JSArrayOfint = () => (JSArrayOfint = dart.constFn(_interceptors.JSArray$(core.int)))();
+  let intTovoid = () => (intTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [core.int])))();
+  let VoidTovoid = () => (VoidTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [])))();
+  regress_20840_test.SomeClass = class SomeClass extends core.Object {
+    new() {
+      this.someField = null;
+      JSArrayOfint().of([1])[dartx.forEach](dart.fn(o => this.someMethod(), intTovoid()));
+      this.someField = new core.Object();
+    }
+    someMethod() {
+      if (this.someField != null) {
+        dart.throw("FAIL");
+      }
+    }
+  };
+  dart.setSignature(regress_20840_test.SomeClass, {
+    constructors: () => ({new: dart.definiteFunctionType(regress_20840_test.SomeClass, [])}),
+    methods: () => ({someMethod: dart.definiteFunctionType(dart.void, [])})
+  });
+  regress_20840_test.main = function() {
+    new regress_20840_test.SomeClass();
+  };
+  dart.fn(regress_20840_test.main, VoidTovoid());
+  // Exports:
+  exports.regress_20840_test = regress_20840_test;
+});