[parser] Add support for private fields in TypeScript (#10483)

* [parser] Add support for private fields in TypeScript

* Fix flow

packages/babel-parser/src/plugins/typescript/index.js

@@ -1868,7 +1868,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     parsePostMemberNameModifiers(
-      methodOrProp: N.ClassMethod | N.ClassProperty,
+      methodOrProp: N.ClassMethod | N.ClassProperty | N.ClassPrivateProperty,
     ): void {
       const optional = this.eat(tt.question);
       if (optional) methodOrProp.optional = true;
@@ -2007,16 +2007,45 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       if (typeParameters) node.typeParameters = typeParameters;
     }
 
-    parseClassProperty(node: N.ClassProperty): N.ClassProperty {
+    parseClassPropertyAnnotation(
+      node: N.ClassProperty | N.ClassPrivateProperty,
+    ): void {
       if (!node.optional && this.eat(tt.bang)) {
         node.definite = true;
       }
 
       const type = this.tsTryParseTypeAnnotation();
       if (type) node.typeAnnotation = type;
+    }
+
+    parseClassProperty(node: N.ClassProperty): N.ClassProperty {
+      this.parseClassPropertyAnnotation(node);
       return super.parseClassProperty(node);
     }
 
+    parseClassPrivateProperty(
+      node: N.ClassPrivateProperty,
+    ): N.ClassPrivateProperty {
+      // $FlowIgnore
+      if (node.abstract) {
+        this.raise(
+          node.start,
+          "Private elements cannot have the 'abstract' modifier.",
+        );
+      }
+
+      // $FlowIgnore
+      if (node.accessibility) {
+        this.raise(
+          node.start,
+          `Private elements cannot have an accessibility modifier ('${node.accessibility}')`,
+        );
+      }
+
+      this.parseClassPropertyAnnotation(node);
+      return super.parseClassPrivateProperty(node);
+    }
+
     pushClassMethod(
       classBody: N.ClassBody,
       method: N.ClassMethod,

packages/babel-parser/src/types.js

@@ -762,7 +762,14 @@ export type ClassPrivateProperty = NodeBase & {
   value: ?Expression, // TODO: Not in spec that this is nullable.
   static: boolean,
   computed: false,
-  typeAnnotation?: ?TypeAnnotation, // TODO: Not in spec
+
+  // Flow and Typescript
+  typeAnnotation?: ?TypeAnnotationBase,
+
+  // TypeScript only
+  optional?: true,
+  definite?: true,
+  readonly?: true,
 };
 
 export type OptClassDeclaration = ClassBase &