[vm] Fix notifications of code observers from background compiler

This CL fixes crash in vm/cc/DartAPI_DartInitializeCallsCodeObserver
test after kernel service is switched to bytecode (so background
compilations are more likely to happen).

Change-Id: Idfc97f16f8658c953628ba0a878b1d6dd2d46186
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113134
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/vm/object.cc

@@ -15053,6 +15053,7 @@ void Code::NotifyCodeObservers(const Function& function,
                                const Code& code,
                                bool optimized) {
 #if !defined(PRODUCT)
+  ASSERT(!function.IsNull());
   ASSERT(!Thread::Current()->IsAtSafepoint());
   // Calling ToLibNamePrefixedQualifiedCString is very expensive,
   // try to avoid it.
@@ -15067,6 +15068,8 @@ void Code::NotifyCodeObservers(const char* name,
                                const Code& code,
                                bool optimized) {
 #if !defined(PRODUCT)
+  ASSERT(name != nullptr);
+  ASSERT(!code.IsNull());
   ASSERT(!Thread::Current()->IsAtSafepoint());
   if (CodeObservers::AreActive()) {
     const auto& instrs = Instructions::Handle(code.instructions());

runtime/vm/stub_code.cc

@@ -210,7 +210,7 @@ RawCode* StubCode::GetAllocationStubForClass(const Class& cls) {
 
       // We notify code observers after finalizing the code in order to be
       // outside a [SafepointOperationScope].
-      Code::NotifyCodeObservers(nullptr, stub, /*optimized=*/false);
+      Code::NotifyCodeObservers(name, stub, /*optimized=*/false);
     }
 #ifndef PRODUCT
     if (FLAG_support_disassembler && FLAG_disassemble_stubs) {