Avoid deprecated List() constructor. (#415)

Enable prefer_collection_literals lint.

analysis_options.yaml

@@ -18,3 +18,4 @@ linter:
     - test_types_in_equals
     - unrelated_type_equality_checks
     - valid_regexps
+    - prefer_collection_literals

lib/src/client/transport/xhr_transport.dart

@@ -158,7 +158,7 @@ class XhrTransportStream implements GrpcTransportStream {
 class XhrClientConnection extends ClientConnection {
   final Uri uri;
 
-  final Set<XhrTransportStream> _requests = Set<XhrTransportStream>();
+  final _requests = <XhrTransportStream>{};
 
   XhrClientConnection(this.uri);
 

test/client_tests/client_interceptor_test.dart

@@ -23,7 +23,7 @@ class FakeInterceptor implements ClientInterceptor {
   int _unary = 0;
   int _streaming = 0;
 
-  static final List<InterceptorInvocation> _invocations = new List();
+  static final _invocations = <InterceptorInvocation>[];
 
   FakeInterceptor(this._id);