fix: privateFieldsAsProperties global counter (#15389)

When babel-helpers are inlined, classPrivateFieldLooseKey()
defines a global variable `id` and initialized it to zero
in every generated file. This can lead to name clashes
between #-private members in subclass and superclass,
which again leads to incorrect semantics.
In the new solution, instead of trying to produce a
globally unique private property name string, a Symbol
is created. The helper function classPrivateFieldLooseKey()
is no longer used, but kept for backwards-compatibility.
In environments where Symbol is not available (Internet
Explorer), a polyfill must be included.

For details and discussion of other solutions, see
#15389

For easier review, the test output updates follow in
a separate commit.

packages/babel-helper-create-class-features-plugin/src/fields.ts

@@ -58,15 +58,14 @@ export function buildPrivateNamesMap(props: PropPath[]) {
 export function buildPrivateNamesNodes(
   privateNamesMap: PrivateNamesMap,
   privateFieldsAsProperties: boolean,
-  state: File,
 ) {
   const initNodes: t.Statement[] = [];
 
   for (const [name, value] of privateNamesMap) {
     // When the privateFieldsAsProperties assumption is enabled,
     // both static and instance fields are transpiled using a
     // secret non-enumerable property. Hence, we also need to generate that
-    // key (using the classPrivateFieldLooseKey helper).
+    // key, using a Symbol.
     // In spec mode, only instance fields need a "private name" initializer
     // because static fields are directly assigned to a variable in the
     // buildPrivateStaticFieldInitSpec function.
@@ -77,9 +76,7 @@ export function buildPrivateNamesNodes(
     let init: t.Expression;
 
     if (privateFieldsAsProperties) {
-      init = t.callExpression(state.addHelper("classPrivateFieldLooseKey"), [
-        t.stringLiteral(name),
-      ]);
+      init = t.callExpression(t.identifier("Symbol"), [t.stringLiteral(name)]);
     } else if (!isStatic) {
       init = t.newExpression(
         t.identifier(!isMethod || isAccessor ? "WeakMap" : "WeakSet"),