Make background optimized compilation default true

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index 0a207de71a5df68920f4bbeb5741de86c09a8019..6c931c39066268d3eaad5e3756566275817a1bfc 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -30,7 +30,7 @@ namespace dart {
 DEFINE_FLAG(bool, deoptimize_alot, false,
     "Deoptimizes all live frames when we are about to return to Dart code from"
     " native entries.");
-DEFINE_FLAG(bool, background_compilation, false,
+DEFINE_FLAG(bool, background_compilation, true,
     "Run optimizing compilation in background");
 DEFINE_FLAG(int, max_subtype_cache_entries, 100,
     "Maximum number of subtype cache entries (number of checks cached).");