Add tests for `warnOnce` utility

src/sidebar/util/test/warn-once-test.js

+'use strict';
+
+const warnOnce = require('../warn-once');
+
+describe('warnOnce', () => {
+  beforeEach(() => {
+    sinon.stub(console, 'warn');
+    warnOnce.reset();
+  });
+
+  afterEach(() => {
+    console.warn.restore();
+  });
+
+  it('outputs a warning only the first time a given string is passed', () => {
+    warnOnce('something is fishy');
+    assert.calledWith(console.warn, 'something is fishy');
+
+    console.warn.reset();
+    warnOnce('something is fishy');
+    assert.notCalled(console.warn);
+
+    warnOnce('something else is wrong');
+    assert.calledWith(console.warn, 'something else is wrong');
+  });
+});

src/sidebar/util/warn-once.js

 'use strict';
 
-const shownWarnings = {};
+let shownWarnings = {};
 
 /**
  * Log a warning if it has not already been reported.
  *
+ * This is useful to avoid spamming the console if a warning is emitted in a
+ * context that may be called frequently.
+ *
  * @param {string} warning
  */
 function warnOnce(warning) {
@@ -15,4 +18,8 @@ function warnOnce(warning) {
   shownWarnings[warning] = true;
 }
 
+warnOnce.reset = () => {
+  shownWarnings = {};
+};
+
 module.exports = warnOnce;