API: incorrect declarations in types.ts

The --strictNullChecks PR changed some declarations in types.ts to avoid frequent assertions in the compiler's code. Unfortunately these changes make it really unsafe for API users because they no longer contain the actually nullable types: https://github.com/Microsoft/TypeScript/pull/22088#discussion_r184149465 https://github.com/Microsoft/TypeScript/pull/22088#discussion_r184155027

• ts.Node#parent is no longer optional (which is definitely wrong for SourceFile)
• ts.Symbol#declarations and ts.Symbol.valueDeclaration are no longer optional, but can still be undefined
• ts.Type#symbol is no longer optional, but I guess this comment is still up to date: // Symbol associated with type (if any)

These changes should either be reverted or replaced in the published declaration files.
/cc @andy-ms @weswigham

types.ts の宣言から始め、strictNullChecks の変更に関するリンク先の議論を確認してください。ts.Node#parent、ts.Symbol#declarations、ts.Symbol.valueDeclaration、ts.Type#symbol の公開済みの宣言ファイルを確認してください。これらの宣言が、issue に記載された nullable または存在しない値を正しく許容していれば完了です。