Fixed weird non-sequential indices for serialisation of `SourceFile` type.
Fixed weird non-sequential indices for serialisation of `SourceFile` type.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust