Handle SyntaxErrors after new-lines specially

In some cases, when we handle SyntaxErrors we need to ensure that the
column number is correct for a 1-indexed report. In some cases, we also
need to account for the fact that the SyntaxError has happened "after" a
new-line. To extract and alter the row and column numbers, we've moved
the logic to a private static method on the FileChecker object to avoid
an overly complex method.

Closes #237

tests/fixtures/example-code/invalid-syntax.py

@@ -0,0 +1 @@
+foo(

tests/unit/test_file_checker.py

@@ -0,0 +1,25 @@
+"""Unit tests for the FileChecker class."""
+import mock
+
+from flake8 import checker
+
+
+@mock.patch('flake8.processor.FileProcessor')
+def test_run_ast_checks_handles_SyntaxErrors(FileProcessor):
+    """Stress our SyntaxError handling.
+
+    Related to: https://gitlab.com/pycqa/flake8/issues/237
+    """
+    processor = mock.Mock(lines=[])
+    FileProcessor.return_value = processor
+    processor.build_ast.side_effect = SyntaxError('Failed to build ast',
+                                                  ('', 1, 5, 'foo(\n'))
+    file_checker = checker.FileChecker(__file__, checks={}, options=object())
+
+    with mock.patch.object(file_checker, 'report') as report:
+        file_checker.run_ast_checks()
+
+        report.assert_called_once_with(
+            'E999', 1, 3,
+            'SyntaxError: Failed to build ast',
+        )