Support choosing channel to download SDK components from.

__tests__/channel-id-mapper.test.ts

@@ -0,0 +1,17 @@
+import * as mapper from '../src/channel-id-mapper';
+
+describe('channel id mapper tests', () => {
+  it('Throws if channelName is unknown', () => {
+    const func = () => {
+      mapper.getChannelId('unknown-channel');
+    };
+    expect(func).toThrowError(`Unexpected channel name: 'unknown-channel'.`);
+  });
+
+  it('Returns expected channelId from channelName', () => {
+    expect(mapper.getChannelId('stable')).toBe(0);
+    expect(mapper.getChannelId('beta')).toBe(1);
+    expect(mapper.getChannelId('dev')).toBe(2);
+    expect(mapper.getChannelId('canary')).toBe(3);
+  });
+});