Add support for class private methods (#703)

* Add support for class private methods

This commit adds parser support for the TC39 Stage 2 Private Methods
proposal.

This commit also changes "key" in ClassPrivateProperty from an
Identifier to a PrivateName, as well as disallowing #constructor as a
valid private field name.

* Add tests for string literal get/set/async

These should be treated as regular methods and not special get/set/async
behaviour.

* Add tests for class private methods

This also removes a test from the Test262 whitelist that failed before
the changes for private methods support and now passes.

* Modify class private prop tests for PrivateName

* Add class private prop tests for #constructor

* Fix existing ASI test case failure

src/plugins/flow.js

@@ -1526,7 +1526,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     // parse type parameters for class methods
-    parseClassMethod(
+    pushClassMethod(
       classBody: N.ClassBody,
       method: N.ClassMethod,
       isGenerator: boolean,
@@ -1541,7 +1541,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         method.typeParameters = this.flowParseTypeParameterDeclaration();
       }
 
-      super.parseClassMethod(
+      super.pushClassMethod(
         classBody,
         method,
         isGenerator,
@@ -1550,6 +1550,23 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       );
     }
 
+    pushClassPrivateMethod(
+      classBody: N.ClassBody,
+      method: N.ClassPrivateMethod,
+      isGenerator: boolean,
+      isAsync: boolean,
+    ): void {
+      if (method.variance) {
+        this.unexpected(method.variance.start);
+      }
+      delete method.variance;
+      if (this.isRelational("<")) {
+        method.typeParameters = this.flowParseTypeParameterDeclaration();
+      }
+
+      super.pushClassPrivateMethod(classBody, method, isGenerator, isAsync);
+    }
+
     // parse a the super class type parameters and implements
     parseClassSuper(node: N.Class): void {
       super.parseClassSuper(node);