Support choosing channel to download SDK components from.

src/input-validator.ts

@@ -1,6 +1,7 @@
 export const MIN_API_LEVEL = 15;
 export const VALID_TARGETS: Array<string> = ['default', 'google_apis', 'google_apis_playstore', 'android-wear', 'android-wear-cn', 'android-tv', 'google-tv'];
 export const VALID_ARCHS: Array<string> = ['x86', 'x86_64', 'arm64-v8a'];
+export const VALID_CHANNELS: Array<string> = ['stable', 'beta', 'dev', 'canary'];
 
 export function checkApiLevel(apiLevel: string): void {
   if (isNaN(Number(apiLevel)) || !Number.isInteger(Number(apiLevel))) {
@@ -23,6 +24,12 @@ export function checkArch(arch: string): void {
   }
 }
 
+export function checkChannel(channel: string): void {
+  if (!VALID_CHANNELS.includes(channel)) {
+    throw new Error(`Value for input.channel '${channel}' is unknown. Supported options: ${VALID_CHANNELS}.`);
+  }
+}
+
 export function checkForceAvdCreation(forceAvdCreation: string): void {
   if (!isValidBoolean(forceAvdCreation)) {
     throw new Error(`Input for input.force-avd-creation should be either 'true' or 'false'.`);