Closure should capture the narrowed type when closure is created with in a narrowed environment

[rdhananjaya]

Description:

import ballerina/io;
public function main() {
    string? x = "hello";

    if (x is string) {
        var f = function() {
                    x = (); // static type of `x` should be `string` but it's `string?` now.
                };
        f();
        string kk = "--" + x + "--"; // x is no longer a string
        io:println(kk);
    }
}

Steps to reproduce:

Affected Versions:

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[MaryamZi]

Related spec issue - ballerina-platform/ballerina-spec#1043.