Do not display import-related errors after module-level always false assert

mypy/semanal_pass1.py

@@ -74,6 +74,10 @@ def visit_file(self, file: MypyFile, fnam: str, mod_id: str, options: Options) -
                     if last.end_line is not None:
                         # We are on a Python version recent enough to support end lines.
                         self.skipped_lines |= set(range(next_def.line, last.end_line + 1))
+                    # To avoid running a new visitor here, let's assume nobody does
+                    # `assert sys.platform == 'foo'; import unknown` on one line
+                    # at top level.
+                    file.imports = [i for i in file.imports if i.line < next_def.line]
                 del file.defs[i + 1 :]
                 break
         file.skipped_lines = self.skipped_lines

test-data/unit/check-incremental.test

@@ -6886,3 +6886,12 @@ class A:
 [out]
 [out2]
 main:3: error: Too few arguments
+
+[case testUnreachableAfterToplevelAssertImportThirdParty]
+# flags: --platform unknown
+import sys
+assert sys.platform == 'linux'
+import does_not_exist
+[builtins fixtures/ops.pyi]
+[out]
+[out2]

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: