code: private void addHeaderCode(Node script) {
      script.addChildToFront(createConditionalObjectDecl(JS_INSTRUMENTATION_OBJECT_NAME, script));
  
      // Make subsequent usages of "window" and "window.top" work in a Web Worker context.
      script.addChildToFront(
          compiler.parseSyntheticCode(
              "if (!self.window) { self.window = self; self.window.top = self; }")
          .removeFirstChild()
          .useSourceInfoIfMissingFromForTree(script));
    }
docs: Creates the js code to be added to source. This code declares and
  initializes the variables required for collection of coverage data.