Fix @babel/generator does not print decorators of private properties (#12081)

packages/babel-generator/src/generators/classes.js

@@ -101,6 +101,7 @@ export function ClassProperty(node: Object) {
 }
 
 export function ClassPrivateProperty(node: Object) {
+  this.printJoin(node.decorators, node);
   if (node.static) {
     this.word("static");
     this.space();

packages/babel-generator/test/fixtures/decorators/private-props/input.js

@@ -0,0 +1,7 @@
+class A {
+  @dec
+  foo;
+
+  @dec
+  #bar;
+}

packages/babel-generator/test/fixtures/decorators/private-props/options.json

@@ -0,0 +1,9 @@
+{
+  "plugins": [
+    ["decorators", { "decoratorsBeforeExport": false }],
+    "classProperties",
+    "classPrivateProperties",
+    "classPrivateMethods"
+  ],
+  "decoratorsBeforeExport": true
+}

packages/babel-generator/test/fixtures/decorators/private-props/output.js

@@ -0,0 +1,6 @@
+class A {
+  @dec
+  foo;
+  @dec
+  #bar;
+}