Work around runtime engine deficiencies.

sdk/lib/_internal/lib/foreign_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library _foreign_helper;
 
 /**
  * Emits a JavaScript code fragment parameterized by arguments.
  *
  * Hash characters `#` in the [codeTemplate] are replaced in left-to-right order
@@ skipping 237 matching lines @@
  *     var foo;
  *
  *     main() {
  *       JS_EFFECT((_){ foo = _; })
  *     }
  *
  * TODO(sra): Replace this hack with something to mark the volatile or
  * externally initialized elements.
  */
 void JS_EFFECT(Function code) { code(null); }
+
+/**
+ * Use this class for creating constants that hold JavaScript code.
+ * For example:
+ *
+ * const constant = JS_CONST('typeof window != "undefined");
+ *
+ * This code will generate:
+ * $.JS_CONST_1 = typeof window != "undefined";
+ */
+class JS_CONST {
+  String code;
+  const JS_CONST(this.code);
+}