[vm] Cleanup todo - assert isolate_group is available when compiling.

If there is current thread, assert there is isolate_group too.

TEST=ci

Change-Id: Id4bce1091084b707b6b671038ec3377ded6791c6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/402427
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/kernel_isolate.cc

@@ -836,15 +836,10 @@ class KernelCompilationRequest : public ValueObject {
     dart_embed_sources.type = Dart_CObject_kBool;
     dart_embed_sources.value.as_bool = embed_sources;
 
-    // TODO(aam): Assert that isolate exists once we move CompileAndReadScript
-    // compilation logic out of CreateIsolateAndSetupHelper and into
-    // IsolateSetupHelper in main.cc.
     auto thread = Thread::Current();
     auto isolate_group = thread != nullptr ? thread->isolate_group() : nullptr;
+    ASSERT((thread == nullptr) || (isolate_group != nullptr));
 
-    if (incremental_compile) {
-      ASSERT(isolate_group != nullptr);
-    }
     Dart_CObject isolate_id;
     isolate_id.type = Dart_CObject_kInt64;
     isolate_id.value.as_int64 = isolate_group != nullptr