Add invariant.js

Author: migueloller

src/build/__tests__/invariant-test.js

@@ -0,0 +1,10 @@
+/* @flow */
+
+import invariant from '../invariant';
+
+describe('invariant()', () => {
+  test('works', () => {
+    invariant(true, '...');
+    expect(() => invariant(false, 'Boom!')).toThrowError('Boom!');
+  });
+});

src/build/invariant.js

@@ -0,0 +1,7 @@
+/* @flow */
+
+export default function invariant(condition: mixed, message: string) {
+  if (!condition) {
+    throw new Error(message);
+  }
+}