Add data structures for chart plugin system (#6028)

* add unit tests

* add test structure

* add unit tests for Registry

* add LoaderRegistry unit test

* add unit test for makeSingleton

* add type check

* add plugin data structures

* simplify API

* add preset tests

* update test message

* fix lint

* update makeSingleton

* update Plugin, Preset and unit test

* revise Registry code

* update unit test, add remove function

* update test

* update unit test

* update plugin unit test

* add .keys(), .entries() and .entriesAsPromise()

* update test description

superset/assets/spec/javascripts/utils/makeSingleton_spec.js

@@ -0,0 +1,38 @@
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import makeSingleton from '../../../src/utils/makeSingleton';
+
+describe('makeSingleton()', () => {
+  class Dog {
+    constructor(name) {
+      this.name = name;
+    }
+    sit() {
+      this.isSitting = true;
+    }
+  }
+  describe('makeSingleton(BaseClass)', () => {
+    const getInstance = makeSingleton(Dog);
+
+    it('returns a function for getting singleton instance of a given base class', () => {
+      expect(getInstance).to.be.a('Function');
+      expect(getInstance()).to.be.instanceOf(Dog);
+    });
+    it('returned function returns same instance across all calls', () => {
+      expect(getInstance()).to.equal(getInstance());
+    });
+  });
+  describe('makeSingleton(BaseClass, ...args)', () => {
+    const getInstance = makeSingleton(Dog, 'Doug');
+
+    it('returns a function for getting singleton instance of a given base class constructed with the given arguments', () => {
+      expect(getInstance).to.be.a('Function');
+      expect(getInstance()).to.be.instanceOf(Dog);
+      expect(getInstance().name).to.equal('Doug');
+    });
+    it('returned function returns same instance across all calls', () => {
+      expect(getInstance()).to.equal(getInstance());
+    });
+  });
+
+});