test.py: temporarily disable background optimizer in vm-dartk-debug config.

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index 6a1d4d4cbb52ce5b78cf7aefc11e6c1db475b4d8..617a17e4895bb2e253f46fb10f276f28694e2a8f 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -143,6 +143,11 @@ class NoneCompilerConfiguration extends CompilerConfiguration {
     if (useDfe) {
       args.add('--dfe=${buildDir}/gen/kernel-service.dart.snapshot');
       args.add('--kernel-binaries=${buildDir}/patched_sdk');
+      if (_isDebug) {
+        // Temporarily disable background compilation to avoid flaky crashes
+        // (see http://dartbug.com/30016 for details).
+        args.add('--no-background-compilation');
+      }
     }
     if (_isChecked) {
       args.add('--enable_asserts');