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/ChartPlugin_spec.js

@@ -0,0 +1,42 @@
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import ChartPlugin from '../../../../src/visualizations/core/models/ChartPlugin';
+import ChartMetadata from '../../../../src/visualizations/core/models/ChartMetadata';
+
+describe('ChartPlugin', () => {
+  const metadata = new ChartMetadata({});
+
+  it('exists', () => {
+    expect(ChartPlugin).to.not.equal(undefined);
+  });
+
+  describe('new ChartPlugin()', () => {
+    it('creates a new plugin', () => {
+      const plugin = new ChartPlugin({
+        metadata,
+        Chart() {},
+      });
+      expect(plugin).to.be.instanceof(ChartPlugin);
+    });
+    it('throws an error if metadata is not specified', () => {
+      expect(() => new ChartPlugin()).to.throw(Error);
+    });
+    it('throws an error if none of Chart or loadChart is specified', () => {
+      expect(() => new ChartPlugin({ metadata })).to.throw(Error);
+    });
+  });
+
+  describe('.register(key)', () => {
+    const plugin = new ChartPlugin({
+      metadata,
+      Chart() {},
+    });
+    it('throws an error if key is not provided', () => {
+      expect(() => plugin.register()).to.throw(Error);
+      expect(() => plugin.configure({ key: 'abc' }).register()).to.not.throw(Error);
+    });
+    it('returns itself', () => {
+      expect(plugin.configure({ key: 'abc' }).register()).to.equal(plugin);
+    });
+  });
+});