Initial design for background compilation

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index 665191594e575f62d16268796dc49709577d789d..309f2700190fed73711f98cf7f441b73f97b79b7 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -46,6 +46,7 @@ DEFINE_FLAG(bool, trace_inlining_intervals, false,
     "Inlining interval diagnostics");
 DEFINE_FLAG(bool, use_megamorphic_stub, true, "Out of line megamorphic lookup");
 
+DECLARE_FLAG(bool, background_compilation);
 DECLARE_FLAG(bool, code_comments);
 DECLARE_FLAG(bool, deoptimize_alot);
 DECLARE_FLAG(int, deoptimize_every);
@@ -114,6 +115,10 @@ static void PrecompilationModeHandler(bool value) {
     FLAG_link_natives_lazily = true;
     FLAG_fields_may_be_reset = true;
     FLAG_allow_absolute_addresses = false;
+
+    // Background compilation relies on two-stage compilation pipeline,
+    // while precompilation has only one.
+    FLAG_background_compilation = false;
   }
 }