Minor fixes for background compilation

runtime/vm/stub_code.cc

Index: runtime/vm/stub_code.cc
diff --git a/runtime/vm/stub_code.cc b/runtime/vm/stub_code.cc
index f7116e320371bc3d51e99a1beef1747e0462a4cb..38ad9a3181a7af6e38032237b02eb96211aa9249 100644
--- a/runtime/vm/stub_code.cc
+++ b/runtime/vm/stub_code.cc
@@ -136,6 +136,11 @@ RawCode* StubCode::GetAllocationStubForClass(const Class& cls) {
       // executable).
       {
         SafepointOperationScope safepoint_scope(thread);
+        stub = cls.allocation_stub();
+        // Check if stub was already generated.
+        if (!stub.IsNull()) {
+          return stub.raw();
+        }
         // Do not Garbage collect during this stage and instead allow the
         // heap to grow.
         NoHeapGrowthControlScope no_growth_control;