"tokenPosTable" can potentially be null

[dstaley]

Today when using the coverage library to check test coverage on a library I'm writing, I ran into the following error:

NoSuchMethodError: The method 'map' was called on null.
Receiver: null
Tried calling: map<List<int>>(Closure: (dynamic) => List<int>)
dart:core                                                                                              Object.noSuchMethod
package:vm_service_client/src/script.dart 181:14                                                       new VMScript._
package:vm_service_client/src/script.dart 62:11                                                        VMScriptRef.load
===== asynchronous gap ===========================
dart:async                                                                                             _AsyncAwaitCompleter.completeError
package:vm_service_client/src/script.dart                                                              VMScriptRef.load
===== asynchronous gap ===========================
dart:async                                                                                             _asyncThenWrapperHelper
package:vm_service_client/src/script.dart                                                              VMScriptRef.load
package:coverage/src/collect.dart 85:30                                                                _getCoverageJson
===== asynchronous gap ===========================
dart:async                                                                                             _asyncThenWrapperHelper
package:coverage/src/collect.dart                                                                      _getCoverageJson
package:coverage/src/collect.dart 51:26                                                                _getAllCoverage
===== asynchronous gap ===========================
dart:async                                                                                             _asyncThenWrapperHelper
package:coverage/src/collect.dart                                                                      _getAllCoverage
package:coverage/src/collect.dart 35:18                                                                collect
===== asynchronous gap ===========================
dart:async                                                                                             _asyncThenWrapperHelper
package:coverage/src/collect.dart                                                                      collect
/Users/dylanstaley/.pub-cache/hosted/pub.dartlang.org/coverage-0.12.3/bin/collect_coverage.dart 22:26  main.<fn>

I did a bit of digging, and the error comes from this line:

  VMScript._(Scope scope, Map json)
      : klass = newVMClassRef(scope, json["class"]),
        size = json["size"],
        library = newVMLibraryRef(scope, json["library"]),
        source = json["source"],
        _tokenPositions = (json["tokenPosTable"] as List) // <<<<
            .map((sublist) => (sublist as List).cast<int>())
            .toList(),
        super._(scope, json);

I found where the json Map was coming from and added a print statement. Sure enough, the Map doesn't have a tokenPosTable key:

{
    type: Script,
    class: {
        type: @Class,
        fixedId: true,
        id: classes/13,
        name: Script
    },
    size: 112,
    id: objects/32,
    uri: org-dartlang-sdk:///runtime/lib/string_buffer_patch.dart,
    _kind: kernel,
    _loadTime: 1545336218821,
    lineOffset: 0,
    columnOffset: 0
}

I'm a good bit out of my comfort zone with this one, but if there's any info I can provide that would be helpful please let me know!