Check in codegen test expectations.

test/codegen/expect/language/issue15720_test.js

Index: test/codegen/expect/language/issue15720_test.js
diff --git a/test/codegen/expect/language/issue15720_test.js b/test/codegen/expect/language/issue15720_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..e2c86712caf8b4a8cf25b048bb316bd012e7faaf
--- /dev/null
+++ b/test/codegen/expect/language/issue15720_test.js
@@ -0,0 +1,35 @@
+dart_library.library('language/issue15720_test', null, /* Imports */[
+  'dart_sdk'
+], function load__issue15720_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 issue15720_test = Object.create(null);
+  let SetOfB = () => (SetOfB = dart.constFn(core.Set$(issue15720_test.B)))();
+  let JSArrayOfB = () => (JSArrayOfB = dart.constFn(_interceptors.JSArray$(issue15720_test.B)))();
+  let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic])))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  issue15720_test.B = class B extends core.Object {};
+  issue15720_test.confuse = function(x) {
+    if (dart.equals(new core.DateTime.now(), 42)) return issue15720_test.confuse(x);
+    return x;
+  };
+  dart.fn(issue15720_test.confuse, dynamicTodynamic());
+  issue15720_test.main = function() {
+    let set = SetOfB().from(JSArrayOfB().of([]));
+    issue15720_test.confuse(499);
+    issue15720_test.confuse(set);
+    let t1 = new issue15720_test.B();
+    let t2 = new issue15720_test.B();
+    let t3 = new issue15720_test.B();
+    let t4 = new issue15720_test.B();
+    set.addAll(JSArrayOfB().of([t1, t2, t3, t4]));
+    issue15720_test.confuse(7);
+    set.addAll(JSArrayOfB().of([t1, t2, t3, t4]));
+  };
+  dart.fn(issue15720_test.main, VoidTodynamic());
+  // Exports:
+  exports.issue15720_test = issue15720_test;
+});