Remove the legacy --additional-compiler-flags option from test.py

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index 54abb58f65c5d8f6c7b26a321e48503a88f927be..e02cffb3c19e6ff7b43d74226576a330723f0bcb 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -708,12 +708,6 @@ class StandardTestSuite extends TestSuite {
 
     case 'dart2dart':
       var compilerArguments = new List.from(args);
-      var additionalFlags =
-          configuration['additional-compiler-flags'].split(' ');
-      for (final flag in additionalFlags) {
-        if (flag.isEmpty) continue;
-        compilerArguments.add(flag);
-      }
       compilerArguments.add('--output-type=dart');
       String tempDir = createOutputDirectory(info.filePath, '');
       compilerArguments.add('--out=$tempDir/out.dart');