code: public void setEnvironment(String value) {
      switch (value) {
        case "BROWSER":
          this.environment = CompilerOptions.Environment.BROWSER;
          break;
        case "CUSTOM":
          this.environment = CompilerOptions.Environment.CUSTOM;
          break;
        default:
          throw new BuildException(
              "Unrecognized 'environment' option value (" + value + ")");
      }
    }
docs: Set the environment which determines the builtin extern set.
  @param value The name of the environment.
  (BROWSER, CUSTOM).