attempt to fix potential race condition

Author: jakemac53

pkgs/test_core/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.6.5-wip
 
 * Increase SDK constraint to ^3.4.0.
+* Fix race condition when compiling multiple tests at once.
 
 ## 0.6.4
 

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

@@ -113,8 +113,8 @@ class _TestCompilerForLanguageVersion {
 
     try {
       if (_frontendServerClient == null) {
-        if (await testCache.exists()) {
-          await testCache.copy(_outputDill.path);
+        if (testCache.existsSync()) {
+          testCache.copySync(_outputDill.path);
         }
         compilerOutput = await _createCompiler(tempFile.uri);
       } else {