build/lib/monaco-api.ts/DeclarationResolver/getDeclarationSourceFile
class DeclarationResolver:	public getDeclarationSourceFile(moduleId: string): ts.SourceFile | null {
		if (this._sourceFileCache[moduleId]) {
			// Since we cannot trust file watching to invalidate the cache, check also the mtime
			const fileName = this._getFileName(moduleId);
			const mtime = this._fsProvider.statSync(fileName).mtime.getTime();
			if (this._sourceFileCache[moduleId]!.mtime !== mtime) {
				this._sourceFileCache[moduleId] = null;
			}
		}
		if (!this._sourceFileCache[moduleId]) {
			this._sourceFileCache[moduleId] = this._getDeclarationSourceFile(moduleId);
		}
		return this._sourceFileCache[moduleId] ? this._sourceFileCache[moduleId]!.sourceFile : null;
	}