fix(core): distribute externs for testability API (#16179)

Workaround for #11119

PR Close #16179

packages/core/src/testability/testability.externs.js

@@ -0,0 +1,23 @@
+/** @externs */
+
+/** @record @struct */
+function PublicTestability() {}
+
+/**
+ * @return {?}
+ */
+PublicTestability.prototype.isStable = function() {};
+
+/**
+ * @param {?} callback
+ * @return {?}
+ */
+PublicTestability.prototype.whenStable = function(callback) {};
+
+/**
+ * @param {?} using
+ * @param {?} provider
+ * @param {?} exactMatch
+ * @return {?}
+ */
+PublicTestability.prototype.findProviders = function(using, provider, exactMatch) {};