anchor.diff:    ### The safe navigation operator ( ?. ) and null property paths
   
     The Angular **safe navigation operator (`?.`)** is a fluent and convenient way to guard against null and undefined values in property paths.
  -  Here it is, protecting against a view render failure if the `currentHero` is null.
  +  Here it is used to protect against against a view render failure if the `currentHero` is null:
   +makeExample('template-syntax/ts/app/app.component.html', 'safe-2')(format=".")
   
   block dart-safe-nav-op
  
positive.code: +makeExample('template-syntax/ts/app/app.component.html', 'safe-1')(format=".")
  :marked
    The view still renders but the displayed value is blank; we see only "The title is" with nothing after it.
    That is reasonable behavior. At least the app doesn't crash.
  
    Suppose the template expression involves a property path, as in this next example
    where we’re displaying the `firstName` of a null hero.
  
  code-example(language="html").
    The null hero's name is {{nullHero.firstName}}
  
negative.code:     The `json` pipe is particularly helpful for debugging our bindings:
    +makeExample('template-syntax/ts/app/app.component.html', 'pipes-json')(format=".")
    :marked
      The generated output would look something like this:
    code-example(language="json").
      { "firstName": "Hercules", "lastName": "Son of Zeus",
        "birthdate": "1970-02-25T08:00:00.000Z",
  