diff --git a/babel.config.js b/babel.config.js
index 44324e7..265fdcc 100644
--- a/babel.config.js
+++ b/babel.config.js
@@ -43,11 +43,29 @@
     onlyRemoveTypeImports: true,
   };
 
+  const addImportExtensionOptions = {
+    extension: DEV || TEST ? 'ts' : 'js',
+  };
+
   // Options for the module-resolver plugin.
   // Used for resolving source files during development.
-  let resolverOptions = {
+  const resolverOptions = {
     alias: {
-      '^@annotator/(.+)$': '@annotator/\\1/src/index.ts',
+      ...(DEV || TEST
+        ? {
+            '^@annotator/(.+)$': '@annotator/\\1/src/index.ts',
+          }
+        : null),
+      // TODO: Remove after babel/babel#8462 ships.
+      '^@babel/runtime-corejs3/core-js/(.+)$':
+        '@babel/runtime-corejs3/core-js/\\1.js',
+      '^@babel/runtime-corejs3/core-js-stable/(.+)$':
+        '@babel/runtime-corejs3/core-js-stable/\\1.js',
+      '^@babel/runtime-corejs3/helpers/(.+)$':
+        '@babel/runtime-corejs3/helpers/\\1.js',
+      '^@babel/runtime-corejs3/regenerator$':
+        '@babel/runtime-corejs3/regenerator/index.js',
+      extensions: ['.js', '.ts'],
     },
   };
 
@@ -56,14 +74,16 @@
     // Use corejs version 3.
     corejs: { version: 3 },
     // Use helpers formatted for the target environment.
-    useESModules: !CJS && !TEST,
+    // TODO: Re-enable after babel/babel#8462 ships.
+    // useESModules: !CJS && !TEST,
   };
 
   return {
     plugins: [
       ['@babel/transform-runtime', runtimeOptions],
       ...(TEST ? ['istanbul'] : []),
-      ...(DEV ? [['module-resolver', resolverOptions]] : []),
+      ['add-import-extension', addImportExtensionOptions],
+      ['module-resolver', resolverOptions],
       'preserve-comment-header',
     ],
     presets: [
diff --git a/package.json b/package.json
index c5fb3e2..913aa56 100644
--- a/package.json
+++ b/package.json
@@ -46,6 +46,7 @@
     "@typescript-eslint/parser": "^3.7.0",
     "ajv": "^6.11.0",
     "babel-loader": "^8.0.5",
+    "babel-plugin-add-import-extension": "^1.4.1",
     "babel-plugin-istanbul": "^6.0.0",
     "babel-plugin-module-resolver": "^4.0.0",
     "babel-plugin-preserve-comment-header": "^1.0.1",
diff --git a/yarn.lock b/yarn.lock
index 07bcebc..259c26b 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -192,6 +192,11 @@
   resolved "https://registry.yarnpkg.com/@babel/helper-plugin-utils/-/helper-plugin-utils-7.10.1.tgz#ec5a5cf0eec925b66c60580328b122c01230a127"
   integrity sha512-fvoGeXt0bJc7VMWZGCAEBEMo/HAjW2mP8apF5eXK0wSqwLAVHAISCWRoLMBMUs2kqeaG77jltVqu4Hn8Egl3nA==
 
+"@babel/helper-plugin-utils@^7.10.4":
+  version "7.10.4"
+  resolved "https://registry.yarnpkg.com/@babel/helper-plugin-utils/-/helper-plugin-utils-7.10.4.tgz#2f75a831269d4f677de49986dff59927533cf375"
+  integrity sha512-O4KCvQA6lLiMU9l2eawBPMf1xPP8xPfB3iEQw150hOVTqj/rfXz0ThTb4HEzqQfs2Bmo5Ay8BzxfzVtBrr9dVg==
+
 "@babel/helper-regex@^7.10.1":
   version "7.10.1"
   resolved "https://registry.yarnpkg.com/@babel/helper-regex/-/helper-regex-7.10.1.tgz#021cf1a7ba99822f993222a001cc3fec83255b96"
@@ -2424,6 +2429,13 @@
     mkdirp "^0.5.1"
     pify "^4.0.1"
 
+babel-plugin-add-import-extension@^1.4.1:
+  version "1.4.1"
+  resolved "https://registry.yarnpkg.com/babel-plugin-add-import-extension/-/babel-plugin-add-import-extension-1.4.1.tgz#d31b919ecbbd052aa3df210eca0b9f0153df770b"
+  integrity sha512-owxjRte+y2QB0YPCv3hWFNLF3Y1S7eEEZCCGo48TzRPM6zEzYsvxtOyx1h95Ghw1gEcl1UnOFIs5CY3Kd4OLYg==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.10.4"
+
 babel-plugin-dynamic-import-node@^2.3.3:
   version "2.3.3"
   resolved "https://registry.yarnpkg.com/babel-plugin-dynamic-import-node/-/babel-plugin-dynamic-import-node-2.3.3.tgz#84fda19c976ec5c6defef57f9427b3def66e17a3"