diff --git a/lib/babel-packages.js.flow b/lib/babel-packages.js.flow
index 100ba525af..d6d45648e6 100644
--- a/lib/babel-packages.js.flow
+++ b/lib/babel-packages.js.flow
@@ -116,3 +116,20 @@ declare module "@babel/code-frame" {
     opts?: Options
   ): string;
 }
+
+declare module "@babel/helper-validator-identifier" {
+  declare function isReservedWord(word: string, inModule: boolean): boolean;
+  declare function isStrictReservedWord(
+    word: string,
+    inModule: boolean
+  ): boolean;
+  declare function isStrictBindOnlyReservedWord(word: string): boolean;
+  declare function isStrictBindReservedWord(
+    word: string,
+    inModule: boolean
+  ): boolean;
+  declare function isKeyword(word: string): boolean;
+  declare function isIdentifierStart(code: number): boolean;
+  declare function isIdentifierChar(code: number): boolean;
+  declare function isIdentifierName(name: string): boolean;
+}
diff --git a/packages/babel-helper-validator-identifier/src/identifier.js b/packages/babel-helper-validator-identifier/src/identifier.ts
similarity index 99%
rename from packages/babel-helper-validator-identifier/src/identifier.js
rename to packages/babel-helper-validator-identifier/src/identifier.ts
index 5088281bbf..0a25c7a9ab 100644
--- a/packages/babel-helper-validator-identifier/src/identifier.js
+++ b/packages/babel-helper-validator-identifier/src/identifier.ts
@@ -1,5 +1,3 @@
-// @flow
-
 import * as charCodes from "charcodes";
 
 // ## Character categories
@@ -38,7 +36,7 @@ const astralIdentifierCodes = [509,0,227,0,150,4,294,9,1368,2,2,1,6,3,41,2,5,0,1
 // This has a complexity linear to the value of the code. The
 // assumption is that looking up astral identifier characters is
 // rare.
-function isInAstralSet(code: number, set: $ReadOnlyArray<number>): boolean {
+function isInAstralSet(code: number, set: readonly number[]): boolean {
   let pos = 0x10000;
   for (let i = 0, length = set.length; i < length; i += 2) {
     pos += set[i];
diff --git a/packages/babel-helper-validator-identifier/src/index.js b/packages/babel-helper-validator-identifier/src/index.ts
similarity index 100%
rename from packages/babel-helper-validator-identifier/src/index.js
rename to packages/babel-helper-validator-identifier/src/index.ts
diff --git a/packages/babel-helper-validator-identifier/src/keyword.js b/packages/babel-helper-validator-identifier/src/keyword.ts
similarity index 99%
rename from packages/babel-helper-validator-identifier/src/keyword.js
rename to packages/babel-helper-validator-identifier/src/keyword.ts
index d062275cc0..55f7b70344 100644
--- a/packages/babel-helper-validator-identifier/src/keyword.js
+++ b/packages/babel-helper-validator-identifier/src/keyword.ts
@@ -1,5 +1,3 @@
-// @flow
-
 const reservedWords = {
   keyword: [
     "break",
diff --git a/packages/babel-types/src/definitions/core.ts b/packages/babel-types/src/definitions/core.ts
index 572fa6d711..ace2ae9f9f 100644
--- a/packages/babel-types/src/definitions/core.ts
+++ b/packages/babel-types/src/definitions/core.ts
@@ -522,7 +522,7 @@ defineType("Identifier", {
       // Ideally we should call isStrictReservedWord if this node is a descendant
       // of a block in strict mode. Also, we should pass the inModule option so
       // we can disable "await" in module.
-      (isKeyword(node.name) || isReservedWord(node.name)) &&
+      (isKeyword(node.name) || isReservedWord(node.name, false)) &&
       // Even if "this" is a keyword, we are using the Identifier
       // node to represent it.
       node.name !== "this"
diff --git a/packages/babel-types/src/validators/isValidIdentifier.ts b/packages/babel-types/src/validators/isValidIdentifier.ts
index 1fd56898d5..46d9fe0456 100644
--- a/packages/babel-types/src/validators/isValidIdentifier.ts
+++ b/packages/babel-types/src/validators/isValidIdentifier.ts
@@ -15,10 +15,8 @@ export default function isValidIdentifier(
   if (typeof name !== "string") return false;
 
   if (reserved) {
-    if (isKeyword(name) || isStrictReservedWord(name)) {
-      return false;
-    } else if (name === "await") {
-      // invalid in module, valid in script; better be safe (see #4952)
+    // "await" is invalid in module, valid in script; better be safe (see #4952)
+    if (isKeyword(name) || isStrictReservedWord(name, true)) {
       return false;
     }
   }