diff --git a/test/unit/helpers/foundation.js b/test/unit/helpers/foundation.js
index 9337d695efd..620272c8a50 100644
--- a/test/unit/helpers/foundation.js
+++ b/test/unit/helpers/foundation.js
@@ -24,44 +24,134 @@
 import {assert} from 'chai';
 import td from 'testdouble';
 
-// Sanity tests to ensure the following:
-// - Default adapters contain functions
-// - All expected adapter functions are accounted for
-// - Invoking any of the default methods does not throw an error.
-// Every foundation test suite include this verification.
-export function verifyDefaultAdapter(FoundationClass, expectedMethods) {
+/**
+ * Sanity tests to ensure the following:
+ * - Default adapters contain functions
+ * - All expected adapter functions are accounted for
+ * - Invoking any of the default methods does not throw an error.
+ * Every foundation test suite include this verification.
+ * @param {!F.} FoundationClass
+ * @param {!Array<string>} expectedMethodNames
+ * @template F
+ */
+export function verifyDefaultAdapter(FoundationClass, expectedMethodNames) {
   const {defaultAdapter} = FoundationClass;
-  const methods = Object.keys(defaultAdapter).filter((k) => typeof defaultAdapter[k] === 'function');
+  const adapterKeys = Object.keys(defaultAdapter);
+  const actualMethodNames = adapterKeys.filter((key) => typeof defaultAdapter[key] === 'function');
+
+  assert.equal(actualMethodNames.length, adapterKeys.length, 'Every adapter key must be a function');
 
-  assert.equal(methods.length, Object.keys(defaultAdapter).length, 'Every adapter key must be a function');
   // Test for equality without requiring that the array be in a specific order
-  assert.deepEqual(methods.slice().sort(), expectedMethods.slice().sort());
+  const actualArray = actualMethodNames.slice().sort();
+  const expectedArray = expectedMethodNames.slice().sort();
+  assert.deepEqual(actualArray, expectedArray, getUnequalArrayMessage(actualArray, expectedArray));
+
   // Test default methods
-  methods.forEach((m) => assert.doesNotThrow(defaultAdapter[m]));
+  actualMethodNames.forEach((m) => assert.doesNotThrow(defaultAdapter[m]));
 }
 
-// Returns an object that intercepts calls to an adapter method used to register event handlers, and adds
-// it to that object where the key is the event name and the value is the function being used. This is the
-// preferred way of testing interaction handlers.
-//
-// ```javascript
-// test('#init adds a click listener which adds a "foo" class', (t) => {
-//   const {foundation, mockAdapter} = setupTest();
-//   const handlers = captureHandlers(mockAdapter, 'registerInteractionHandler');
-//   foundation.init();
-//   handlers.click(/* you can pass event info in here */ {type: 'click'});
-//   t.doesNotThrow(() => td.verify(mockAdapter.addClass('foo')));
-//   t.end();
-// });
-// ```
-//
-// Note that `handlerCaptureMethod` _must_ have a signature of `(string, EventListener) => any` in order to
-// be effective.
-export function captureHandlers(adapter, handlerCaptureMethod) {
+/**
+ * Returns an object that intercepts calls to an adapter method used to register event handlers, and adds
+ * it to that object where the key is the event name and the value is the function being used. This is the
+ * preferred way of testing interaction handlers.
+ *
+ * ```javascript
+ * test('#init adds a click listener which adds a "foo" class', (t) => {
+ *   const {foundation, mockAdapter} = setupTest();
+ *   const handlers = captureHandlers(mockAdapter, 'registerInteractionHandler');
+ *   foundation.init();
+ *   handlers.click(/* you can pass event info in here *\/ {type: 'click'});
+ *   t.doesNotThrow(() => td.verify(mockAdapter.addClass('foo')));
+ *   t.end();
+ * });
+ * ```
+ *
+ * Note that `handlerCaptureMethodName` _must_ have a signature of `(string, EventListener) => any` in order to
+ * be effective.
+ *
+ * @param {!A} adapter
+ * @param {string} handlerCaptureMethodName
+ * @template A
+ */
+export function captureHandlers(adapter, handlerCaptureMethodName) {
   const {isA} = td.matchers;
   const handlers = {};
-  td.when(adapter[handlerCaptureMethod](isA(String), isA(Function))).thenDo((type, handler) => {
+  td.when(adapter[handlerCaptureMethodName](isA(String), isA(Function))).thenDo((type, handler) => {
     handlers[type] = (evtInfo = {}) => handler(Object.assign({type}, evtInfo));
   });
   return handlers;
 }
+
+/**
+ * @param {!Array<string>} actualArray
+ * @param {!Array<string>} expectedArray
+ * @return {string}
+ */
+function getUnequalArrayMessage(actualArray, expectedArray) {
+  /**
+   * @param {!Array<string>} values
+   * @param {string} singularName
+   * @return {string}
+   */
+  const format = (values, singularName) => {
+    const count = values.length;
+    if (count === 0) {
+      return '';
+    }
+    const plural = count === 1 ? '' : 's';
+    const str = values.join(', ');
+    return `${count} ${singularName}${plural}: ${str}`;
+  };
+
+  /**
+   * @param {!Set<string>} actualSet
+   * @param {!Set<string>} expectedSet
+   * @return {string}
+   */
+  const getAddedStr = (actualSet, expectedSet) => {
+    const addedArray = [];
+    actualSet.forEach((val) => {
+      if (!expectedSet.has(val)) {
+        addedArray.push(val);
+      }
+    });
+    return format(addedArray, 'unexpected method');
+  };
+
+  /**
+   * @param {!Set<string>} actualSet
+   * @param {!Set<string>} expectedSet
+   * @return {string}
+   */
+  const getRemovedStr = (actualSet, expectedSet) => {
+    const removedArray = [];
+    expectedSet.forEach((val) => {
+      if (!actualSet.has(val)) {
+        removedArray.push(val);
+      }
+    });
+    return format(removedArray, 'missing method');
+  };
+
+  /**
+   * @param {!Array<string>} array
+   * @return {!Set<string>}
+   */
+  const toSet = (array) => {
+    const set = new Set();
+    array.forEach((value) => set.add(value));
+    return set;
+  };
+
+  const actualSet = toSet(actualArray);
+  const expectedSet = toSet(expectedArray);
+  const addedStr = getAddedStr(actualSet, expectedSet);
+  const removedStr = getRemovedStr(actualSet, expectedSet);
+  const messages = [addedStr, removedStr].filter((val) => val.length > 0);
+
+  if (messages.length === 0) {
+    return '';
+  }
+
+  return `Found ${messages.join('; ')}`;
+}
diff --git a/test/unit/helpers/setup.js b/test/unit/helpers/setup.js
index f3bd7d50a42..1be0eedb475 100644
--- a/test/unit/helpers/setup.js
+++ b/test/unit/helpers/setup.js
@@ -23,8 +23,15 @@
 
 import td from 'testdouble';
 
-// Returns a foundation configured to use a mock object with the same api as a default adapter,
-// as well as that adapter itself.
+/**
+ * Returns a foundation configured to use a mock object with the same API as a default adapter,
+ * as well as that adapter itself.
+ * The trailing `.` in the `@param` type below is intentional: It indicates a reference to the class itself instead of
+ * an instance of the class.
+ * See https://youtrack.jetbrains.com/issue/WEB-10214#focus=streamItem-27-1305930-0-0
+ * @param {!MDCFoundation.} FoundationClass
+ * @return {{mockAdapter: !Object, foundation: !MDCFoundation}}
+ */
 export function setupFoundationTest(FoundationClass) {
   const mockAdapter = td.object(FoundationClass.defaultAdapter);
   const foundation = new FoundationClass(mockAdapter);
diff --git a/test/unit/index.js b/test/unit/index.js
index a3a4c8519ff..fbef0b91abe 100644
--- a/test/unit/index.js
+++ b/test/unit/index.js
@@ -23,5 +23,6 @@
 
 /** @fileoverview Bootstraps the test bundle for karma-webpack. */
 
-const testsContext = require.context('.', true, /\.test\.js$/);
+// https://github.com/webpack/docs/wiki/context#requirecontext
+const testsContext = require.context(/* directory */ '.', /* useSubdirectories */ true, /\.test\.js$/);
 testsContext.keys().forEach(testsContext);