unclear error from dart2js checked mode - dart

Question

My original error was An attempt was made to use an object that is not, or is no longer, usable. After dart2js runs with the default settings it's pretty difficult to read and I was having trouble debugging the output, so I added these options to my pubspec.yaml:
- $dart2js:
checked: true
sourceMaps: true
verbose: true
minify: false
I was expecting something to break, but I was hoping the error message would be clear enough that I could work it out, but I understand the error I'm getting.
The new error I'm getting is:
Error:
dart<.wrapException()
dart<._rootHandleUncaughtError_closure.call$0()
dart<._microtaskLoop()
dart<._startMicrotaskLoop<()
dart<._AsyncRun__initializeScheduleImmediate_internalCallback.call$1<()
dart<.invokeClosure_closure0.call$0()
dart<._IsolateContext.eval$1()
dart<._callInIsolate()
dart<.invokeClosure<()
dart<.convertDartClosureToJS/$function</<()
It looks like dart is having some trouble converting a closure to a js function. I'd have thought this would be done when dart2js is run and not while the application is running in the browser.
My code doesn't have any explicit closures, but I do use async await. Am I right in assuming that those would be converted into closures?
Just in case it's relevant, I'm also using Angular2.
Update
It appears that my problems are being caused by my use of the HttpRequest.request() from dart:html.
HttpRequest res = await HttpRequest.request( "${API_URL}/login",
method : "POST",
responseType: "json",
mimeType : "application/json",
sendData : JSON.encode( req_data ) );
print( res.status );
If I remove checked mode and build this is giving me the An attempt was made to use an object that is not, or is no longer, usable error.

Answer 1

The API is better, more intuitive.
For example your request :
var client = new BrowserClient();
var response = await client.post("${API_URL}/login",
body: JSON.encode( req_data ));
var data = JSON.decode(response.body);
client.close();
And you could probably see in the parameters if you want more precision in the way you want to define your request.

Answer 2

It seems that I have fixed the problem by changing my use of the HttpRequest class.
From:
HttpRequest res = await HttpRequest.request( "${API_URL}/login",
method : "POST",
responseType: "json",
mimeType : "application/json",
sendData : JSON.encode( req_data ) );
print( res.status );
To:
HttpRequest res = new HttpRequest()
..open( "POST", "${API_URL}/login" );
req
..onLoadEnd.listen( ( ProgressEvent e ) {
print( res.status );
})
..send( JSON.encode( req_data ) );
I'm not sure what the actual problem was, but this way I don't get either the InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable or the longer dart error:
Error:
dart<.wrapException()
dart<._rootHandleUncaughtError_closure.call$0()
dart<._microtaskLoop()
dart<._startMicrotaskLoop<()
dart<._AsyncRun__initializeScheduleImmediate_internalCallback.call$1<()
dart<.invokeClosure_closure0.call$0()
dart<._IsolateContext.eval$1()
dart<._callInIsolate()
dart<.invokeClosure<()
dart<.convertDartClosureToJS/$function</<()
My guess is that dart2js is hving some trouble with async await "implicit" closures and is much happier when I use an "explicit here's a function that closes over it's environment" closure.
Thanks to Günter Zöchbauer for the help.

Answer 3

You should use http package instead
https://pub.dartlang.org/packages/http