@typescript/vfs: createVirtualTypeScriptEnvironment stores a duplicate SourceFile per file that the language service never reads

Page URL: https://github.com/microsoft/TypeScript-Website/blob/v2/packages/typescript-vfs/src/index.ts (@typescript/vfs)

Issue:

createVirtualTypeScriptEnvironment permanently stores a second parsed AST for every file, roughly doubling per-file memory.

Where the duplicates are created:

• The env's createFile/updateFile parse a SourceFile (ts.createSourceFile / ts.updateSourceFile) and pass it down to createVirtualCompilerHost's updateFile, which saves it in its internal sourceFiles map.
• The language service never reads that map — its host's getScriptSnapshot reads text from sys, and the language service parses and caches its own SourceFiles from those snapshots.

So the ASTs in the compiler-host map are write-only: every file that goes through env.createFile/env.updateFile is held as two full ASTs (the language service's copy plus the map's copy), and the map's copy is never released for the lifetime of the env.

Start in packages/typescript-vfs/src/index.ts and trace createVirtualTypeScriptEnvironment's createFile/updateFile calls into createVirtualCompilerHost's updateFile. Confirm how the sourceFiles map is populated and whether the language service reads it. Done means eliminating the unused duplicate SourceFile storage without changing language-service behavior; run the package's existing tests if available.