[enums] Parse exporting enums

Summary:
Adds support for:
```
export enum A {}
```
and
```
export default enum A {}
```

Reviewed By: gabelevi

Differential Revision: D17105742

fbshipit-source-id: a1a7140fb9be6d896de8c0dea2c3abd93904cf30

Author: gkz

packages/flow-parser/test/custom_ast_types.js

@@ -96,6 +96,7 @@ def("ExportDefaultDeclaration")
       def("FunctionDeclaration"), // TODO: should be NullableFunctionDeclaration
       def("VariableDeclaration"),
       def("InterfaceDeclaration"),
+      def("EnumDeclaration"),
       def("TypeAlias"),
       def("OpaqueType"),
       def("Expression")))

src/parser/statement_parser.ml

@@ -987,6 +987,9 @@ module Statement
             (* export default class foo { ... } *)
             let _class = Object.class_declaration env decorators in
             Declaration _class
+          else if Peek.token env = T_ENUM then
+            (* export default enum foo { ... } *)
+            Declaration (Declaration.enum_declaration env)
           else
             (* export default [assignment expression]; *)
             let expr = Parse.assignment env in
@@ -1068,7 +1071,8 @@ module Statement
     (* not using Peek.is_function here because it would guard all of the
       * cases *)
     | T_ASYNC
-    | T_FUNCTION ->
+    | T_FUNCTION
+    | T_ENUM ->
         let open Statement.ExportNamedDeclaration in
         let stmt = Parse.statement_list_item env ~decorators:decorators in
         let names = Statement.(
@@ -1080,6 +1084,7 @@ module Statement
             ) [] declarations
           | (loc, ClassDeclaration { Class.id = Some id; _; })
           | (loc, FunctionDeclaration { Function.id = Some id; _; })
+          | (loc, EnumDeclaration {EnumDeclaration.id; _})
             -> [Flow_ast_utils.ident_of_source (loc, extract_ident_name id)]
           | (loc, ClassDeclaration { Class.id = None; _; }) ->
             error_at env (loc, Parse_error.ExportNamelessClass);

src/parser/test/flow/enums/enum-export.js

@@ -0,0 +1,3 @@
+export enum A {}
+
+export default enum B {}

src/parser/test/flow/enums/enum-export.options.json

@@ -0,0 +1,3 @@
+{
+  "enums": true
+}

src/parser/test/flow/enums/enum-export.tree.json

@@ -0,0 +1,62 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":3,"column":24}},
+  "range":[0,42],
+  "body":[
+    {
+      "type":"ExportNamedDeclaration",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":16}},
+      "range":[0,16],
+      "declaration":{
+        "type":"EnumDeclaration",
+        "loc":{"source":null,"start":{"line":1,"column":7},"end":{"line":1,"column":16}},
+        "range":[7,16],
+        "id":{
+          "type":"Identifier",
+          "loc":{"source":null,"start":{"line":1,"column":12},"end":{"line":1,"column":13}},
+          "range":[12,13],
+          "name":"A",
+          "typeAnnotation":null,
+          "optional":false
+        },
+        "body":{
+          "type":"EnumStringBody",
+          "loc":{"source":null,"start":{"line":1,"column":7},"end":{"line":1,"column":16}},
+          "range":[7,16],
+          "members":[],
+          "explicitType":false
+        }
+      },
+      "specifiers":[],
+      "source":null,
+      "exportKind":"value"
+    },
+    {
+      "type":"ExportDefaultDeclaration",
+      "loc":{"source":null,"start":{"line":3,"column":0},"end":{"line":3,"column":24}},
+      "range":[18,42],
+      "declaration":{
+        "type":"EnumDeclaration",
+        "loc":{"source":null,"start":{"line":3,"column":15},"end":{"line":3,"column":24}},
+        "range":[33,42],
+        "id":{
+          "type":"Identifier",
+          "loc":{"source":null,"start":{"line":3,"column":20},"end":{"line":3,"column":21}},
+          "range":[38,39],
+          "name":"B",
+          "typeAnnotation":null,
+          "optional":false
+        },
+        "body":{
+          "type":"EnumStringBody",
+          "loc":{"source":null,"start":{"line":3,"column":15},"end":{"line":3,"column":24}},
+          "range":[33,42],
+          "members":[],
+          "explicitType":false
+        }
+      },
+      "exportKind":"value"
+    }
+  ],
+  "comments":[]
+}