Do not display import-not-found after module-level always false assert

Author: sterliakov

mypy/errors.py

@@ -633,6 +633,8 @@ def add_error_info(self, info: ErrorInfo) -> None:
                         return
             if file in self.ignored_files:
                 return
+            if file in self.skipped_lines and set(lines) <= self.skipped_lines[file]:
+                return
         if info.only_once:
             if info.message in self.only_once_messages:
                 return

test-data/unit/check-unreachable-code.test

@@ -778,6 +778,13 @@ assert sys.platform == 'lol'
 def bar() -> None: pass
 [builtins fixtures/ops.pyi]
 
+[case testUnreachableAfterToplevelAssertImportThirdParty]
+# flags: --platform unknown
+import sys
+assert sys.platform == 'linux'
+import does_not_exist
+[builtins fixtures/ops.pyi]
+
 [case testUnreachableAfterToplevelAssertNotInsideIf]
 import sys
 if sys.version_info[0] >= 2: