Fix null-safety warning in dart.dart (#3514)

lib/src/dart.dart

diff --git a/lib/src/dart.dart b/lib/src/dart.dart
index 0169174..5c3828e 100644
--- a/lib/src/dart.dart
+++ b/lib/src/dart.dart
@@ -186,7 +186,7 @@
     final result = await client.compile();
 
     final highlightedName = log.bold(name);
-    if (result?.errorCount == 0) {
+    if (result.errorCount == 0) {
       log.message('Built $highlightedName.');
       // By using rename we ensure atomicity. An external observer will either
       // see the old or the new snapshot.
@@ -200,7 +200,7 @@
 
       throw ApplicationException(
           log.yellow('Failed to build $highlightedName:\n') +
-              (result?.compilerOutputLines.join('\n') ?? ''));
+              result.compilerOutputLines.join('\n'));
     }
   } finally {
     client?.kill();

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index e44e0e7..4f52414 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -12,7 +12,7 @@
   cli_util: ^0.3.5
   collection: ^1.15.0
   crypto: ^3.0.1
-  frontend_server_client: ^2.0.0
+  frontend_server_client: ^3.0.0
   http: ^0.13.3
   http_multi_server: ^3.0.1
   http_parser: ^4.0.1
@@ -31,6 +31,6 @@
 dev_dependencies:
   lints: ^2.0.0
   shelf_test_handler: ^2.0.0
-  test: ^1.17.3
+  test: ^1.21.5
   test_descriptor: ^2.0.0
   test_process: ^2.0.0