Fix multiple var definition with Undefined
Migrate more checker test cases to new syntax

Some are still disabled.
Migrate more checker test cases to new syntax

Some are still disabled.
Migrate more checker test cases to new syntax

Some are still disabled.
Migrate more checker test cases to new syntax

Some are still disabled.
Migrate more checker test cases to new syntax
Fix type checking casts
Refactor test case
Enable test cases that use casts
Update docstring
Use special AST node for typevar(...)
Fix type checking of type applications
Migrate more checker test cases to new syntax

Some are still disabled.
Add test case headings
Fix test case
Enable test case
Enable test cases
Enable test cases
Enable test cases
Enable some test cases