fix: check native source correctly

[HerrCai0907]

native code's internal path is ~lib/native.ts instead of ~lib.
However, it is more easier to check current source is same as static
_native field.

Fixes # .

Changes proposed in this pull request:
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[CountBleck]

Source._native is lazily initialized. Maybe just fix the line to say LIBRARY_PREFIX + "native.ts" (what Source.native contains verbatim).

You could also do return this == Source.native but I think that defeats the purpose of both lazy-initializing and the optimization you were trying to make.

[HerrCai0907]

Source._native is lazily initialized. Maybe just fix the line to say LIBRARY_PREFIX + "native.ts" (what Source.native contains verbatim).

lazy init is ok. When Source._native is null, it will always return false. which is also correct since native source is never created.

[CountBleck]

Silly me