add class features

experimental/class-features.md

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+
+# Class Features
+
+This experimental extension covers three class features proposals: 
+[Class Fields], [Static Class Features] and [Private Methods].
+
+## ClassBody
+
+```js
+extend interface ClassBody {
+    body: [ MethodDefinition | PropertyDefinition ];
+}
+```
+
+## PropertyDefinition
+
+```js
+interface PropertyDefinition <: Node {
+    type: "PropertyDefinition";
+    key: Expression | PrivateName;
+    value: Expression | null;
+    computed: boolean;
+    static: boolean;
+    private: boolean;
+}
+```
+
+- When `private` is `true`, `computed` must be `false` and `key` must be a `PrivateName`.
+- When `private` is `false`, `key` must be an `Expression` and can not be a `PrivateName`.
+
+## MethodDefinition
+
+```js
+extend interface MethodDefinition {
+    key: Expression | PrivateName;
+    private: boolean;
+}
+```
+
+- When `private` is `true`, `computed` must be `false`, `key` must be a `PrivateName`, `kind` can not be `"constructor"`.
+- When `private` is `false`, `key` must be an `Expression` and can not be a `PrivateName`.
+
+### PrivateName
+
+```js
+interface PrivateName <: Node {
+    type: "PrivateName";
+    name: string;
+}
+```
+
+A private name refers to private class elements. For a private name `#a`, its `name` is `a`.
+
+```js
+extend interface MemberExpression {
+    private: boolean;
+    property: Expression | PrivateName;
+}
+```
+
+- When `private` is `true`, `property` must be a `PrivateName`, `computed` must be `false`.
+- When `private` is `false`, `property` must be an `Expression` and can not be a `PrivateName`.
+- When `object` is a `Super`, `private` must be `false`.
+
+[Class Fields]: https://github.com/tc39/proposal-class-fields
+[Static Class Features]: https://github.com/tc39/proposal-static-class-features/
+[Private Methods]: https://github.com/tc39/proposal-private-methods