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/visualizations/models/Preset_spec.js

@@ -0,0 +1,65 @@
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import Preset from '../../../../src/visualizations/core/models/Preset';
+import Plugin from '../../../../src/visualizations/core/models/Plugin';
+
+describe('Preset', () => {
+  it('exists', () => {
+    expect(Preset).to.not.equal(undefined);
+  });
+
+  describe('new Preset()', () => {
+    it('creates new preset', () => {
+      const preset = new Preset();
+      expect(preset).to.be.instanceOf(Preset);
+    });
+  });
+
+  describe('.register()', () => {
+    it('register all listed presets then plugins', () => {
+      const values = [];
+      class Plugin1 extends Plugin {
+        register() {
+          values.push(1);
+        }
+      }
+      class Plugin2 extends Plugin {
+        register() {
+          values.push(2);
+        }
+      }
+      class Plugin3 extends Plugin {
+        register() {
+          values.push(3);
+        }
+      }
+      class Plugin4 extends Plugin {
+        register() {
+          const { key } = this.config;
+          values.push(key);
+        }
+      }
+
+      const preset1 = new Preset({
+        plugins: [new Plugin1()],
+      });
+      const preset2 = new Preset({
+        plugins: [new Plugin2()],
+      });
+      const preset3 = new Preset({
+        presets: [preset1, preset2],
+        plugins: [
+          new Plugin3(),
+          new Plugin4().configure({ key: 'abc' }),
+        ],
+      });
+      preset3.register();
+      expect(values).to.deep.equal([1, 2, 3, 'abc']);
+    });
+
+    it('returns itself', () => {
+      const preset = new Preset();
+      expect(preset.register()).to.equal(preset);
+    });
+  });
+});