Fixes #378

test/codegen/expect/extensions.js

Index: test/codegen/expect/extensions.js
diff --git a/test/codegen/expect/extensions.js b/test/codegen/expect/extensions.js
new file mode 100644
index 0000000000000000000000000000000000000000..e164782b3cabee6c42c1e2e792ca1a29c3d44610
--- /dev/null
+++ b/test/codegen/expect/extensions.js
@@ -0,0 +1,25 @@
+dart_library.library('extensions', null, /* Imports */[
+  "dart/_runtime",
+  'dart/collection',
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, dart, collection, core) {
+  'use strict';
+  let dartx = dart.dartx;
+  class StringIterable extends collection.IterableBase$(core.String) {
+    StringIterable() {
+      this.iterator = null;
+      super.IterableBase();
+    }
+  }
+  dart.virtualField(StringIterable, 'iterator');
+  dart.setSignature(StringIterable, {});
+  dart.defineExtensionMembers(StringIterable, ['iterator']);
+  function main() {
+    return new StringIterable();
+  }
+  dart.fn(main);
+  // Exports:
+  exports.StringIterable = StringIterable;
+  exports.main = main;
+});