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

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