Add SystemNavigator.pop (flutter/engine#3130)

This method will let us remove the activity service.

engine/src/flutter/shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -51,6 +51,8 @@ public class PlatformPlugin extends JSONMessageListener implements ActivityLifec
             setSystemChromeEnabledSystemUIOverlays(args.getJSONArray(0));
         } else if (method.equals("SystemChrome.setSystemUIOverlayStyle")) {
             setSystemChromeSystemUIOverlayStyle(args.getString(0));
+        } else if (method.equals("SystemNavigator.pop")) {
+            popSystemNavigator();
         } else if (method.equals("PathProvider.getTemporaryDirectory")) {
             return getPathProviderTemporaryDirectory();
         } else if (method.equals("PathProvider.getApplicationDocumentsDirectory")) {
@@ -160,6 +162,10 @@ public class PlatformPlugin extends JSONMessageListener implements ActivityLifec
         // so LIGHT vs DARK effectively isn't supported in Android.
     }
 
+    private void popSystemNavigator() {
+        mActivity.finish();
+    }
+
     private JSONObject getPathProviderTemporaryDirectory() throws JSONException {
         JSONObject result = new JSONObject();
         result.put("path", mActivity.getCacheDir().getPath());

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformPlugin.mm

@@ -56,6 +56,8 @@ using namespace shell;
     [self setSystemChromeEnabledSystemUIOverlays:args.firstObject];
   } else if ([method isEqualToString:@"SystemChrome.setSystemUIOverlayStyle"]) {
     [self setSystemChromeSystemUIOverlayStyle:args.firstObject];
+  } else if ([method isEqualToString:@"SystemNavigator.pop"]) {
+    [self popSystemNavigator];
   } else if ([method isEqualToString:@"PathProvider.getTemporaryDirectory"]) {
     return [self getPathProviderTemporaryDirectory];
   } else if ([method isEqualToString:@"PathProvider.getApplicationDocumentsDirectory"]) {
@@ -160,6 +162,10 @@ using namespace shell;
   }
 }
 
+- (void)popSystemNavigator {
+  // Apple's human user guidelines say not to terminate iOS applications.
+}
+
 - (NSDictionary*)getPathProviderTemporaryDirectory {
   return GetDirectoryOfType(NSCachesDirectory);
 }