Make ArrayExpression validator accept nulls as holes in the array

packages/babel-types/src/definitions/core.js

@@ -12,6 +12,7 @@ import {
 import defineType, {
   assertValueType,
   assertNodeType,
+  assertNodeOrValueType,
   assertEach,
   chain,
   assertOneOf,
@@ -20,7 +21,7 @@ import defineType, {
 defineType("ArrayExpression", {
   fields: {
     elements: {
-      validate: chain(assertValueType("array"), assertEach(assertNodeType("Expression", "SpreadElement")))
+      validate: chain(assertValueType("array"), assertEach(assertNodeOrValueType("null", "Expression", "SpreadElement")))
     }
   },
   visitor: ["elements"],

packages/babel-types/src/definitions/index.js

@@ -61,6 +61,27 @@ export function assertNodeType(...types: Array<string>): Function {
   return validate;
 }
 
+export function assertNodeOrValueType(...types: Array<string>): Function {
+  function validate(node, key, val) {
+    let valid = false;
+
+    for (let type of types) {
+      if (getType(val) === type || t.is(type, val)) {
+        valid = true;
+        break;
+      }
+    }
+
+    if (!valid) {
+      throw new TypeError(`Property ${key} of ${node.type} expected node to be of a type ${JSON.stringify(types)} but instead got ${JSON.stringify(val && val.type)}`);
+    }
+  }
+
+  validate.oneOfNodeOrValueTypes = types;
+
+  return validate;
+}
+
 export function assertValueType(type: string): Function {
   function validate(node, key, val) {
     let valid = getType(val) === type;