RDFLib: read remote turtle files (e.g. from web link)

[tian3rd]

E.g.,

// ...
Graph g = Graph();
g.parse('http://www.w3.org/People/Berners-Lee/card');
// ...

[tian3rd]

For test purpose, we can try to parse this link: http://www.w3.org/2002/07/owl#

[zheyxu]

Made pr #45
The webLink should point to a valid Turtle (.ttl) file to ensure correct parsing.
If the webLink does not point to a .ttl file, it might cause unexpected parsing errors.

[zheyxu]

Check with

  String webLink = 'https://www.w3.org/TR/turtle/examples/example3.ttl';

  // Create a graph to read Turtle file and store info.
  Graph g = Graph();

  // Parse the Turtle file from the web.
  await g.parseTurtleFromWeb(webLink);```

[zheyxu]

The web link must be end with '.ttl'.
await g.parseTurtleFromWeb('https://www.w3.org/2002/07/owl#') does not work in this case.
But if copy and paste content of 'https://www.w3.org/2002/07/owl#' to a local file, and named it example_ttl_6.dart.
The following code works.

String filePath = 'example/sample_ttl_6.ttl';

  // Read file content to a local String

  String fileContents = await File(filePath).readAsStringSync();

  g.parseTurtle(fileContents);

The difference between String webStr = http.get('https://www.w3.org/2002/07/owl#') and String fileContents = await File(filePath).readAsStringSync(); is the format of empty space.