print gen_snapshot exit code; fix a missing type annotation (#4976)

packages/flutter_tools/lib/src/commands/build_aot.dart

@@ -256,7 +256,8 @@ Future<String> _buildAotSnapshot(
 
   RunResult results = await runAsync(genSnapshotCmd);
   if (results.exitCode != 0) {
-    printStatus(results.toString());
+    printError('Dart snapshot generator failed with exit code ${results.exitCode}');
+    printError(results.toString());
     return null;
   }
 

packages/flutter_tools/lib/src/ios/devices.dart

@@ -260,7 +260,7 @@ class IOSDevice extends Device {
           forwardDiagPort = _acquireAndForwardPort(ProtocolDiscovery.kDiagnosticService,
                                                    debuggingOptions.diagnosticPort);
         } else {
-          forwardDiagPort = new Future.value(null);
+          forwardDiagPort = new Future<int>.value(null);
         }
 
         return Future.wait(<Future<int>>[forwardObsPort, forwardDiagPort]);