[vm/concurrency] Speed up debug runs of isolate tests

Running isolates tests in debug mode is currently very slow due to heap
verification and related code on isolate startup & shutdown.

This CL limits those verifications to only run on the first isolate of
an isolate group.

Issue https://github.com/dart-lang/sdk/issues/36097

TEST=Existing test suite.

Change-Id: I1f329bca9e4c1d56ab60f36ffa8b9cc037b818f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199249
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

runtime/vm/dart.cc

@@ -861,7 +861,9 @@ ErrorPtr Dart::InitializeIsolate(const uint8_t* snapshot_data,
   }
 
   Object::VerifyBuiltinVtables();
-  DEBUG_ONLY(IG->heap()->Verify(kForbidMarked));
+  if (T->isolate()->origin_id() == 0) {
+    DEBUG_ONLY(IG->heap()->Verify(kForbidMarked));
+  }
 
 #if defined(DART_PRECOMPILED_RUNTIME)
   const bool kIsAotRuntime = true;

runtime/vm/dart_api_impl.cc

@@ -1499,7 +1499,9 @@ DART_EXPORT void Dart_ShutdownIsolate() {
     StackZone zone(T);
     HandleScope handle_scope(T);
 #if defined(DEBUG)
-    T->isolate_group()->ValidateConstants();
+    if (T->isolate()->origin_id() == 0) {
+      T->isolate_group()->ValidateConstants();
+    }
 #endif
     Dart::RunShutdownCallback();
   }