Omit code guarded by 'if TYPE_CHECKING'

This commit finds a problem that has been observed in read code.

Consider the following code:

```
from typing import TYPE_CHECKING
...
if TYPE_CHECKING
    from a import A, B
    from b import C

...
def f() -> "B":
    ...

def f()
    # Oops! C is acutally used here.
    C()
```

This commit ignores all code that is guarded by TYPE_CHECKING in
order to find mistakes like this. This constant is always False
when mypy is not running anyway.

pyflakes/checker.py

@@ -1833,7 +1833,13 @@ def DICT(self, node):
     def IF(self, node):
         if isinstance(node.test, ast.Tuple) and node.test.elts != []:
             self.report(messages.IfTuple, node)
-        self.handleChildren(node)
+        omit = []
+        # We don't process the body of the if-statement if it is guarded by
+        # if TYPE_CHECKING. This variable is always false when the code is
+        # running.
+        if _is_typing(node.test, 'TYPE_CHECKING', self.scopeStack):
+            omit.append("body")
+        self.handleChildren(node, omit=omit)
 
     IFEXP = IF
 

pyflakes/test/test_imports.py

@@ -1031,6 +1031,14 @@ def test_futureImportStar(self):
         from __future__ import *
         ''', m.FutureFeatureNotDefined)
 
+    def test_ignoresTypingImports(self):
+        """Imports within 'if TYPE_CHECKING' are ignored."""
+        self.flakes('''
+        from typing import TYPE_CHECKING
+        if TYPE_CHECKING:
+            from a import b
+        b()
+        ''', m.UndefinedName)
 
 class TestSpecialAll(TestCase):
     """