[Bridge] Add support for JS async functions to RCT_EXPORT_METHOD

Summary:
Adds support for JS async methods and helps guide people writing native modules w.r.t. the callbacks. With this diff, on the native side you write:

```objc
RCT_EXPORT_METHOD(getValueAsync:(NSString *)key
                       resolver:(RCTPromiseResolver)resolve
                       rejecter:(RCTPromiseRejecter)reject)
{
  NSError *error = nil;
  id value = [_nativeDataStore valueForKey:key error:&error];

  // "resolve" and "reject" are automatically defined blocks that take
  // any object (nil is OK) and an NSError, respectively
  if (!error) {
    resolve(value);
  } else {
    reject(error);
  }
}
```

On the JS side, you can write:

```js
var {DemoDataStore} = require('react-native').NativeModules;
DemoDataStore.getValueAsync('sample-key').then((value) => {
  console.log('Got:', value);
}, (error) => {
  console.error(error);
  // "error" is an Error object whose message is the NSError's description.
  // The NSError's code and domain are also set, and the native trace i
Closes https://github.com/facebook/react-native/pull/1232
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/BatchedBridge/BatchingImplementation/BatchedBridgeFactory.js

@@ -19,9 +19,17 @@ var slice = Array.prototype.slice;
 
 var MethodTypes = keyMirror({
   remote: null,
+  remoteAsync: null,
   local: null,
 });
 
+type ErrorData = {
+  message: string;
+  domain: string;
+  code: number;
+  nativeStackIOS?: string;
+};
+
 /**
  * Creates remotely invokable modules.
  */
@@ -36,21 +44,40 @@ var BatchedBridgeFactory = {
    */
   _createBridgedModule: function(messageQueue, moduleConfig, moduleName) {
     var remoteModule = mapObject(moduleConfig.methods, function(methodConfig, memberName) {
-      return methodConfig.type === MethodTypes.local ? null : function() {
-        var lastArg = arguments.length > 0 ? arguments[arguments.length - 1] : null;
-        var secondLastArg = arguments.length > 1 ? arguments[arguments.length - 2] : null;
-        var hasSuccCB = typeof lastArg === 'function';
-        var hasErrorCB = typeof secondLastArg === 'function';
-        hasErrorCB && invariant(
-          hasSuccCB,
-          'Cannot have a non-function arg after a function arg.'
-        );
-        var numCBs = (hasSuccCB ? 1 : 0) + (hasErrorCB ? 1 : 0);
-        var args = slice.call(arguments, 0, arguments.length - numCBs);
-        var onSucc = hasSuccCB ? lastArg : null;
-        var onFail = hasErrorCB ? secondLastArg : null;
-        return messageQueue.call(moduleName, memberName, args, onFail, onSucc);
-      };
+      switch (methodConfig.type) {
+        case MethodTypes.remote:
+          return function() {
+            var lastArg = arguments.length > 0 ? arguments[arguments.length - 1] : null;
+            var secondLastArg = arguments.length > 1 ? arguments[arguments.length - 2] : null;
+            var hasErrorCB = typeof lastArg === 'function';
+            var hasSuccCB = typeof secondLastArg === 'function';
+            hasSuccCB && invariant(
+              hasErrorCB,
+              'Cannot have a non-function arg after a function arg.'
+            );
+            var numCBs = (hasSuccCB ? 1 : 0) + (hasErrorCB ? 1 : 0);
+            var args = slice.call(arguments, 0, arguments.length - numCBs);
+            var onSucc = hasSuccCB ? secondLastArg : null;
+            var onFail = hasErrorCB ? lastArg : null;
+            messageQueue.call(moduleName, memberName, args, onSucc, onFail);
+          };
+
+        case MethodTypes.remoteAsync:
+          return function(...args) {
+            return new Promise((resolve, reject) => {
+              messageQueue.call(moduleName, memberName, args, resolve, (errorData) => {
+                var error = _createErrorFromErrorData(errorData);
+                reject(error);
+              });
+            });
+          };
+
+        case MethodTypes.local:
+          return null;
+
+        default:
+          throw new Error('Unknown bridge method type: ' + methodConfig.type);
+      }
     });
     for (var constName in moduleConfig.constants) {
       warning(!remoteModule[constName], 'saw constant and method named %s', constName);
@@ -59,7 +86,6 @@ var BatchedBridgeFactory = {
     return remoteModule;
   },
 
-
   create: function(MessageQueue, modulesConfig, localModulesConfig) {
     var messageQueue = new MessageQueue(modulesConfig, localModulesConfig);
     return {
@@ -80,4 +106,14 @@ var BatchedBridgeFactory = {
   }
 };
 
+function _createErrorFromErrorData(errorData: ErrorData): Error {
+  var {
+    message,
+    ...extraErrorInfo,
+  } = errorData;
+  var error = new Error(message);
+  error.framesToPop = 1;
+  return Object.assign(error, extraErrorInfo);
+}
+
 module.exports = BatchedBridgeFactory;