Create a custom version of fastparse.py for Python 2 (#2102)

* Add a modified version of fastparse for Python 2.7

This commit adds a modified version of the fastparse module that works
specifically on a Python 2 AST. This allows us to skip the step of first
having to convert a Python 2 AST into a Python 3 AST, which helps boost
performance on large codebases.

* Make mypy start using fastparse 2 when applicable

This commit adjusts parse.py and fastparse.py slightly so that they
defer to fastparse2.py as necessary when encountering Python 2 code.

* Fix bugs with new Python 2 AST converter

This commit fixes a few bugs leftover from the conversion process, which
were mainly remnants of WIP items I forgot to fix.

Author: Michael0x2a

mypy/fastparse.py

@@ -25,9 +25,7 @@
 from mypy.errors import Errors
 
 try:
-    from typed_ast import ast27
     from typed_ast import ast35
-    from typed_ast import conversions
 except ImportError:
     if sys.version_info.minor > 2:
         print('You must install the typed_ast package before you can run mypy'
@@ -59,11 +57,8 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
     """
     is_stub_file = bool(fnam) and fnam.endswith('.pyi')
     try:
-        if pyversion[0] >= 3 or is_stub_file:
-            ast = ast35.parse(source, fnam, 'exec')
-        else:
-            ast2 = ast27.parse(source, fnam, 'exec')
-            ast = conversions.py2to3(ast2)
+        assert pyversion[0] >= 3 or is_stub_file
+        ast = ast35.parse(source, fnam, 'exec')
 
         tree = ASTConverter(pyversion=pyversion,
                             is_stub=is_stub_file,
@@ -802,6 +797,12 @@ class TypeConverter(ast35.NodeTransformer):
     def __init__(self, line: int = -1) -> None:
         self.line = line
 
+    def visit_raw_str(self, s: str) -> Type:
+        # An escape hatch that allows the AST walker in fastparse2 to
+        # directly hook into the Python 3.5 type converter in some cases
+        # without needing to create an intermediary `ast35.Str` object.
+        return parse_type_comment(s.strip(), line=self.line)
+
     def generic_visit(self, node: ast35.AST) -> None:
         raise TypeCommentParseError(TYPE_COMMENT_AST_ERROR, self.line)