src/vs/workbench/api/common/extHostDocuments.ts/ExtHostDocuments/$acceptModelLanguageChanged
class ExtHostDocuments:	public $acceptModelLanguageChanged(uriComponents: UriComponents, newLanguageId: string): void {
		const uri = URI.revive(uriComponents);
		const data = this._documentsAndEditors.getDocument(uri);
		if (!data) {
			throw new Error('unknown document');
		}
		// Treat a language change as a remove + add

		this._onDidRemoveDocument.fire(data.document);
		data._acceptLanguageId(newLanguageId);
		this._onDidAddDocument.fire(data.document);
	}