[parser] Disallow duplicate and undeclared private names (#10456)

* [parser] Add private names tracking to Scope

- Disallow duplicate private names
- Disallow undeclared private names

* Update tests

* Test all possible duplications

* Test undeclared private names

* Better error message for top-level private names

* Fix flow

* Update test262 whitelist

* Update fixtures

* Update flow whitelist

* Remove old output.json

* Move ClassScopeHandler to a separate class

* Make the code readable

packages/babel-parser/src/tokenizer/index.js

@@ -391,14 +391,8 @@ export default class Tokenizer extends LocationParser {
     }
 
     if (
-      (this.hasPlugin("classPrivateProperties") ||
-        this.hasPlugin("classPrivateMethods")) &&
-      this.state.classLevel > 0
-    ) {
-      ++this.state.pos;
-      this.finishToken(tt.hash);
-      return;
-    } else if (
+      this.hasPlugin("classPrivateProperties") ||
+      this.hasPlugin("classPrivateMethods") ||
       this.getPluginOption("pipelineOperator", "proposal") === "smart"
     ) {
       this.finishOp(tt.hash, 1);

packages/babel-parser/src/tokenizer/state.js

@@ -77,9 +77,6 @@ export default class State {
   soloAwait: boolean = false;
   inFSharpPipelineDirectBody: boolean = false;
 
-  // Check whether we are in a (nested) class or not.
-  classLevel: number = 0;
-
   // Labels in scope.
   labels: Array<{
     kind: ?("loop" | "switch"),