schema: generate runtime constants for enum values (#961)

Summary:
We have a `const Reactions` convenience enum in `github/graphql.js`.
That value is useful, but that module is slated to die. This commit
extends our Flow type generation script to include these values.

Test Plan:
Existing unit tests suffice.

wchargin-branch: schema-generate-enums

src/graphql/__snapshots__/generateFlowTypes.test.js.snap

@@ -9,12 +9,20 @@ export type Actor = Human | TalentedChimpanzee;
 
 export type Color = \\"BLUE\\" | \\"GREEN\\" | \\"RED\\";
 
+export const Color$Values: {|
+  +BLUE: \\"BLUE\\",
+  +GREEN: \\"GREEN\\",
+  +RED: \\"RED\\",
+|} = Object.freeze({ BLUE: \\"BLUE\\", GREEN: \\"GREEN\\", RED: \\"RED\\" });
+
 export type DateTime = string;
 
 export type Dollars = number;
 
 export type EmptyEnum = empty;
 
+export const EmptyEnum$Values: {||} = Object.freeze({});
+
 export type EmptyUnion = empty;
 
 export type Human = {|

src/graphql/generateFlowTypes.js

@@ -16,7 +16,7 @@ export default function generateFlowTypes(
   schema: Schema,
   prettierOptions: PrettierOptions
 ): string {
-  const typedefs = [];
+  const definitions = [];
 
   function formatField(field: FieldType): string {
     switch (field.type) {
@@ -66,17 +66,32 @@ export default function generateFlowTypes(
     const type = schema[typename];
     switch (type.type) {
       case "SCALAR":
-        typedefs.push(`export type ${typename} = ${type.representation};`);
+        definitions.push(`export type ${typename} = ${type.representation};`);
         break;
       case "ENUM": {
-        const rhs =
-          Object.keys(type.values).length === 0
-            ? "empty"
-            : Object.keys(type.values)
+        const values = Object.keys(type.values)
           .sort()
-                .map((x) => JSON.stringify(x))
-                .join(" | ");
-        typedefs.push(`export type ${typename} = ${rhs};`);
+          .map((x) => JSON.stringify(x));
+
+        // export type E = "A" | "B";
+        const typeRhs = values.length === 0 ? "empty" : values.join(" | ");
+        definitions.push(`export type ${typename} = ${typeRhs};`);
+
+        // export const E$Values: {|+A: "A", +B: "B"|} = Object.freeze(...);
+        const objectName = `${typename}$Values`;
+        const objectType = [
+          "{|",
+          values.map((x) => `+${x}: ${x}`).join(", "),
+          "|}",
+        ].join("");
+        const objectValue = [
+          "Object.freeze({",
+          values.map((x) => `${x}: ${x}`).join(", "),
+          "})",
+        ].join("");
+        definitions.push(
+          `export const ${objectName}: ${objectType} = ${objectValue};`
+        );
         break;
       }
       case "OBJECT": {
@@ -90,7 +105,7 @@ export default function generateFlowTypes(
           .map((x) => `+${x.fieldname}: ${x.rhs}`)
           .join(", ");
         const rhs = `{|\n${fieldContents}\n|}`;
-        typedefs.push(`export type ${typename} = ${rhs};`);
+        definitions.push(`export type ${typename} = ${rhs};`);
         break;
       }
       case "UNION": {
@@ -100,7 +115,7 @@ export default function generateFlowTypes(
             : Object.keys(type.clauses)
                 .sort()
                 .join(" | ");
-        typedefs.push(`export type ${typename} = ${rhs};`);
+        definitions.push(`export type ${typename} = ${rhs};`);
         break;
       }
       // istanbul ignore next: unreachable per Flow
@@ -112,7 +127,7 @@ export default function generateFlowTypes(
   const rawSource = [
     "// @flow",
     "// Autogenerated file. Do not edit.",
-    ...typedefs,
+    ...definitions,
   ].join("\n\n");
   return prettier.format(rawSource, prettierOptions);
 }

src/plugins/github/graphqlTypes.js

@@ -110,6 +110,20 @@ export type PullRequestReviewState =
   | "DISMISSED"
   | "PENDING";
 
+export const PullRequestReviewState$Values: {|
+  +APPROVED: "APPROVED",
+  +CHANGES_REQUESTED: "CHANGES_REQUESTED",
+  +COMMENTED: "COMMENTED",
+  +DISMISSED: "DISMISSED",
+  +PENDING: "PENDING",
+|} = Object.freeze({
+  APPROVED: "APPROVED",
+  CHANGES_REQUESTED: "CHANGES_REQUESTED",
+  COMMENTED: "COMMENTED",
+  DISMISSED: "DISMISSED",
+  PENDING: "PENDING",
+});
+
 export type Reaction = {|
   +__typename: "Reaction",
   +content: ReactionContent,
@@ -125,6 +139,22 @@ export type ReactionContent =
   | "THUMBS_DOWN"
   | "THUMBS_UP";
 
+export const ReactionContent$Values: {|
+  +CONFUSED: "CONFUSED",
+  +HEART: "HEART",
+  +HOORAY: "HOORAY",
+  +LAUGH: "LAUGH",
+  +THUMBS_DOWN: "THUMBS_DOWN",
+  +THUMBS_UP: "THUMBS_UP",
+|} = Object.freeze({
+  CONFUSED: "CONFUSED",
+  HEART: "HEART",
+  HOORAY: "HOORAY",
+  LAUGH: "LAUGH",
+  THUMBS_DOWN: "THUMBS_DOWN",
+  THUMBS_UP: "THUMBS_UP",
+});
+
 export type Ref = {|
   +__typename: "Ref",
   +id: string,