YapfBear: Disable syntax verification

Syntax verification is supposed to be a private feature in yapf, used
for debugging, but they're still defaulting its flag to True on the
public FormatFile function. We need to set it to False to disable
syntax error checking in yapf (which seems to be an unmaintained
feature of it, since it raises exceptions on async/await and Python 2's
print statement), see discussion at
google/yapf#293

Fixes coala#738

bears/python/YapfBear.py

@@ -142,7 +142,8 @@ def run(self, filename, file,
         with prepare_file(options.splitlines(keepends=True),
                           None) as (file_, fname):
             corrected = FormatFile(filename,
-                                   style_config=fname)[0].splitlines(True)
+                                   style_config=fname,
+                                   verify=False)[0].splitlines(True)
         diffs = Diff.from_string_arrays(file, corrected).split_diff()
         for diff in diffs:
             yield Result(self,

tests/python/YapfBearTest.py

@@ -1,4 +1,6 @@
+import sys
 from queue import Queue
+from unittest.case import skipIf
 
 from bears.python.YapfBear import YapfBear
 from tests.LocalBearTestHelper import LocalBearTestHelper
@@ -22,6 +24,15 @@ def test_valid(self):
                             ["x = {  'a':37,'b':42,\n", "'c':927}\n", '\n',
                              "y = 'hello ''world'\n"], valid=False)
 
+    def test_valid_python_2(self):
+        self.check_validity(self.uut, ['print 1\n'], valid=True)
+
+    @skipIf(sys.version_info < (3, 5), "ast before 3.5 can't parse async def")
+    def test_valid_async(self):
+        self.check_validity(self.uut,
+                            ['async def x():\n', '    pass\n'],
+                            valid=True)
+
     def test_blank_line_after_nested_class_or_def(self):
         self.section.append(Setting('blank_line_before_nested_class_or_def',
                                     True))