Read-only AST types (#1121)

**Potential breaking change: Where previously AST would produce `null`, it now produces `undefined`**

This tightens up the AST types to produce readonly covariant fields, $ReadOnlyArray in a couple places (not AST yet, perhaps in a future PR), and replaces `field?: ?Rule` with `field?: Rule` to exclude the possibility of `null` from `undefined` for optional rules, reducing complexity.

Author: leebyron

src/execution/values.js

@@ -186,7 +186,7 @@ export function getArgumentValues(
  */
 export function getDirectiveValues(
   directiveDef: GraphQLDirective,
-  node: { directives?: ?Array<DirectiveNode> },
+  node: { +directives?: $ReadOnlyArray<DirectiveNode> },
   variableValues?: ?ObjMap<mixed>,
 ): void | { [argument: string]: mixed } {
   const directiveNode =

src/jsutils/find.js

@@ -8,7 +8,7 @@
  */
 
 export default function find<T>(
-  list: Array<T>,
+  list: $ReadOnlyArray<T>,
   predicate: (item: T) => boolean,
 ): ?T {
   for (let i = 0; i < list.length; i++) {

src/jsutils/quotedOrList.js

@@ -12,7 +12,7 @@ const MAX_LENGTH = 5;
 /**
  * Given [ A, B, C ] return '"A", "B", or "C"'.
  */
-export default function quotedOrList(items: Array<string>): string {
+export default function quotedOrList(items: $ReadOnlyArray<string>): string {
   const selected = items.slice(0, MAX_LENGTH);
   return selected
     .map(item => `"${item}"`)

src/jsutils/suggestionList.js

@@ -13,7 +13,7 @@
  */
 export default function suggestionList(
   input: string,
-  options: Array<string>,
+  options: $ReadOnlyArray<string>,
 ): Array<string> {
   const optionsByDistance = Object.create(null);
   const oLength = options.length;

src/language/__tests__/parser-test.js

@@ -221,7 +221,7 @@ describe('Parser', () => {
           kind: Kind.OPERATION_DEFINITION,
           loc: { start: 0, end: 40 },
           operation: 'query',
-          name: null,
+          name: undefined,
           variableDefinitions: [],
           directives: [],
           selectionSet: {
@@ -231,7 +231,7 @@ describe('Parser', () => {
               {
                 kind: Kind.FIELD,
                 loc: { start: 4, end: 38 },
-                alias: null,
+                alias: undefined,
                 name: {
                   kind: Kind.NAME,
                   loc: { start: 4, end: 8 },
@@ -261,28 +261,28 @@ describe('Parser', () => {
                     {
                       kind: Kind.FIELD,
                       loc: { start: 22, end: 24 },
-                      alias: null,
+                      alias: undefined,
                       name: {
                         kind: Kind.NAME,
                         loc: { start: 22, end: 24 },
                         value: 'id',
                       },
                       arguments: [],
                       directives: [],
-                      selectionSet: null,
+                      selectionSet: undefined,
                     },
                     {
                       kind: Kind.FIELD,
                       loc: { start: 30, end: 34 },
-                      alias: null,
+                      alias: undefined,
                       name: {
                         kind: Kind.NAME,
                         loc: { start: 30, end: 34 },
                         value: 'name',
                       },
                       arguments: [],
                       directives: [],
-                      selectionSet: null,
+                      selectionSet: undefined,
                     },
                   ],
                 },
@@ -311,7 +311,7 @@ describe('Parser', () => {
           kind: Kind.OPERATION_DEFINITION,
           loc: { start: 0, end: 29 },
           operation: 'query',
-          name: null,
+          name: undefined,
           variableDefinitions: [],
           directives: [],
           selectionSet: {
@@ -321,7 +321,7 @@ describe('Parser', () => {
               {
                 kind: Kind.FIELD,
                 loc: { start: 10, end: 27 },
-                alias: null,
+                alias: undefined,
                 name: {
                   kind: Kind.NAME,
                   loc: { start: 10, end: 14 },
@@ -336,15 +336,15 @@ describe('Parser', () => {
                     {
                       kind: Kind.FIELD,
                       loc: { start: 21, end: 23 },
-                      alias: null,
+                      alias: undefined,
                       name: {
                         kind: Kind.NAME,
                         loc: { start: 21, end: 23 },
                         value: 'id',
                       },
                       arguments: [],
                       directives: [],
-                      selectionSet: null,
+                      selectionSet: undefined,
                     },
                   ],
                 },

src/language/__tests__/schema-parser-test.js

@@ -399,7 +399,7 @@ type Hello {
                 inputValueNode(
                   nameNode('flag', { start: 22, end: 26 }),
                   typeNode('Boolean', { start: 28, end: 35 }),
-                  null,
+                  undefined,
                   { start: 22, end: 35 },
                 ),
               ],
@@ -481,7 +481,7 @@ type Hello {
                     type: typeNode('String', { start: 31, end: 37 }),
                     loc: { start: 30, end: 38 },
                   },
-                  null,
+                  undefined,
                   { start: 22, end: 38 },
                 ),
               ],
@@ -518,13 +518,13 @@ type Hello {
                 inputValueNode(
                   nameNode('argOne', { start: 22, end: 28 }),
                   typeNode('Boolean', { start: 30, end: 37 }),
-                  null,
+                  undefined,
                   { start: 22, end: 37 },
                 ),
                 inputValueNode(
                   nameNode('argTwo', { start: 39, end: 45 }),
                   typeNode('Int', { start: 47, end: 50 }),
-                  null,
+                  undefined,
                   { start: 39, end: 50 },
                 ),
               ],
@@ -657,7 +657,7 @@ input Hello {
             inputValueNode(
               nameNode('world', { start: 17, end: 22 }),
               typeNode('String', { start: 24, end: 30 }),
-              null,
+              undefined,
               { start: 17, end: 30 },
             ),
           ],