Internal slot properties (#7947)

* Support internalSlots to babel-types and babel-generator

* Parsing support for internal slot properties

* Print internal slots in babel-generator

* Add whitespace before first internal slot property

packages/babylon/test/fixtures/flow/interfaces-module-and-script/1/output.json

@@ -80,6 +80,7 @@
           "callProperties": [],
           "properties": [],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/10/output.json

@@ -235,6 +235,7 @@
             }
           ],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/2/output.json

@@ -114,6 +114,7 @@
           "callProperties": [],
           "properties": [],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/3/output.json

@@ -278,6 +278,7 @@
           "callProperties": [],
           "properties": [],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/4/output.json

@@ -151,6 +151,7 @@
             }
           ],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/5/output.json

@@ -198,6 +198,7 @@
               "variance": null
             }
           ],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/id-reserved-value/output.json

@@ -80,6 +80,7 @@
           "callProperties": [],
           "properties": [],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }

packages/babylon/test/fixtures/flow/interfaces-module-and-script/identifier-named-static-optional/output.json

@@ -133,6 +133,7 @@
             }
           ],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }