dart-lang/js-interop — pull request #81 by a14n (2013-05-28 19:42:33+00:00)
Title: Add a propertyExists() method

Since js.Proxy is implemented using noSuchMethod(), it's not so easy to check if a property exists. This makes it awkward to translate code like this snippet from a Google Drive example:

if (authResult && !authResult.error) {
...
} else {
...
}

Workaround: write your own propertyExists() JavaScript function.