Add invariant module

If Flow sees a function named `invariant` being called, it will behave
as if an exception was thrown inline where the passed condition isn't
true. The is useful for determining unreachable code and maintaining
refinements.

In other words, `invariant(foo, msg)` is equivalent to:

  if (!foo) throw new Error(msg);

in that Flow even refines `foo` as truthy below the condition in both
cases.

Author: mwiencek

.eslintrc.yaml

@@ -27,6 +27,7 @@ globals:
   addColonText: readonly
   hasOwnProp: readonly
   hyphenateTitle: readonly
+  invariant: readonly
   nonEmpty: readonly
   l: readonly
   ln: readonly

root/utility/invariant.js

@@ -0,0 +1,24 @@
+/*
+ * @flow strict
+ * Copyright (C) 2020 MetaBrainz Foundation
+ *
+ * This file is part of MusicBrainz, the open internet music database,
+ * and is licensed under the GPL version 2, or (at your option) any
+ * later version: http://www.gnu.org/licenses/gpl-2.0.txt
+ */
+
+/*
+ * If Flow sees a function named `invariant` being called, it will behave
+ * as if an exception was thrown inline where the passed condition isn't
+ * true. The is useful for determining unreachable code and maintaining
+ * refinements.
+ */
+export default function invariant(
+  cond: mixed,
+  msg?: string = 'Invariant Violation',
+): void {
+  // flowlint sketchy-null-mixed:off
+  if (!cond) {
+    throw new Error(msg);
+  }
+}

root/vars.js

@@ -109,3 +109,6 @@ declare var texp: {
 
 // https://flow.org/en/docs/tips/switch-statement-exhaustiveness/
 declare var exhaustive: (action: empty) => void;
+
+// root/utility/invariant.js
+declare var invariant: (cond: mixed, msg?: string) => empty;

webpack/providePluginConfig.js

@@ -18,12 +18,14 @@ const expandPath = path.resolve(i18nPath, 'expand2react');
 const expandTextPath = path.resolve(i18nPath, 'expand2text');
 const hasOwnPropPath = path.resolve(commonPath, 'utility/hasOwnProp');
 const nonEmptyPath = path.resolve(commonPath, 'utility/nonEmpty');
+const invariantPath = path.resolve(__dirname, '../root/utility/invariant');
 
 module.exports = {
   'addColon': [addColonPath, 'default'],
   'addColonText': [addColonPath, 'addColonText'],
   'hasOwnProp': [hasOwnPropPath, 'default'],
   'hyphenateTitle': [hyphenateTitlePath, 'default'],
+  'invariant': [invariantPath, 'default'],
   'nonEmpty': [nonEmptyPath, 'default'],
 
   'l': [i18nPath, 'l'],