Fix unawaited_return_in_try_block (#2654)

In preparation for new diagnostics, fix existing violations.

pkgs/test_core/lib/src/runner/vm/platform.dart

diff --git a/pkgs/test_core/lib/src/runner/vm/platform.dart b/pkgs/test_core/lib/src/runner/vm/platform.dart
index 1986a85..0f7620e 100644
--- a/pkgs/test_core/lib/src/runner/vm/platform.dart
+++ b/pkgs/test_core/lib/src/runner/vm/platform.dart
@@ -298,14 +298,14 @@
     try {
       var precompiledPath = _config.suiteDefaults.precompiledPath;
       if (precompiledPath != null) {
-        return _spawnPrecompiledIsolate(
+        return await _spawnPrecompiledIsolate(
           path,
           message,
           precompiledPath,
           compiler,
         );
       }
-      return switch (compiler) {
+      return await switch (compiler) {
         Compiler.kernel => _spawnIsolateWithUri(
           await _compileToKernel(path, suiteMetadata),
           message,