Add native_testing library to mock @Native classes

tests/html/js_interop_constructor_name_test_js.js

+(function() {
+
+  // A constructor function with the same name as a HTML element.
+  function HTMLDivElement(a) {
+    this.a = a;
+  }
+
+  HTMLDivElement.prototype.bar = function() {
+    return this.a;
+  }
+
+  HTMLDivElement.prototype.toString = function() {
+    return "HTMLDivElement(" + this.a + ")";
+  }
+
+  self.makeDiv = function(text) {
+    return new HTMLDivElement(text);
+  };
+
+})();