fix strong mode warnings

[dam0vm3nt]

: [error] Type check failed: _createSet (() → Set<dynamic>) is not of type () → Set<int> (package:isolate/registry.dart, line 346, col 34)
[error] Type check failed: _createSet (() → Set<dynamic>) is not of type () → Set<int> (package:isolate/registry.dart, line 373, col 44)
[error] Invalid override. The type of _MultiplexRawReceivePort.handler= (((dynamic) → void) → void) is not a subtype of RawReceivePort.handler= ((Function) → void). (package:isolate/src/raw_receive_port_multiplexer.dart, line 38, col 3)
[warning] Unsound implicit cast from Future<dynamic> to Future<bool> (package:isolate/isolate_runner.dart, line 133, col 12)
[warning] Unsound implicit cast from List<dynamic> to List<Future<dynamic>> (package:isolate/load_balancer.dart, line 101, col 14)
[warning] Unsound implicit cast from List<dynamic> to List<Future<dynamic>> (package:isolate/load_balancer.dart, line 130, col 12)
[warning] Unsound implicit cast from List<dynamic> to List<_LoadBalancerEntry> (package:isolate/load_balancer.dart, line 240, col 14)
[warning] Unsound implicit cast from Future<dynamic> to Future<Capability> (package:isolate/registry.dart, line 128, col 12)
[warning] Unsound implicit cast from Future<dynamic> to Future<bool> (package:isolate/registry.dart, line 150, col 12)
[warning] Unsound implicit cast from List<dynamic> to List<int> (package:isolate/registry.dart, line 164, col 21)
[warning] Unsound implicit cast from Future<dynamic> to Future<List<T>> (package:isolate/registry.dart, line 220, col 12)
[warning] Unsound implicit cast from dynamic to List<int> (package:isolate/registry.dart, line 323, col 18)
[warning] Unsound implicit cast from dynamic to List<int> (package:isolate/registry.dart, line 326, col 21)
[warning] Unsound implicit cast from Future<dynamic> to Future<List<dynamic>> (package:isolate/src/errors.dart, line 89, col 12)
[warning] Unsound implicit cast from Future<dynamic> to Future<List<dynamic>> (package:isolate/src/errors.dart, line 147, col 12)

[cbracken]

Fixed by #7 (and even earlier #6). Will be released in 0.2.3.