Add a prefix splitting.

test/test_prefix.py

@@ -0,0 +1,24 @@
+import pytest
+import parso
+
+
+@pytest.mark.parametrize(('string', 'tokens'), [
+    ('#', ['#']),
+    (' # ', [' ', '# ']),
+    (' # \n', [' ', '# ', '\n']),
+    (' # \f\n', [' ', '# ', '\f', '\n']),
+    ('  \n', ['  ', '\n']),
+    ('  \n ', ['  ', '\n', ' ']),
+    (' \f ', [' ', '\f', ' ']),
+    (' \f ', [' ', '\f', ' ']),
+    (' \r\n', [' ', '\r\n']),
+    ('\\', ['\\']),
+    ('\\\n', ['\\\n']),
+    ('\\\r\n', ['\\\r\n']),
+])
+def test_simple_prefix_splitting(string, tokens):
+    tree = parso.parse(string)
+    leaf = tree.children[0]
+    assert leaf.type == 'endmarker'
+    parsed_tokens = list(leaf._split_prefix())
+    assert [t.value for t in parsed_tokens] == tokens