Add IsMutatorThread to the Thread class and use it instead of MutatorThreadIsCurrentThread

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index 21f2124bc95d9503f61b446ebdba3ab2ea0d2e83..39196c19d2fe142667f2e3931092f0ac8b91f5e3 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -1165,7 +1165,7 @@ RawError* Compiler::CompileOptimizedFunction(Thread* thread,
   // Optimization must happen in non-mutator/Dart thread if background
   // compilation is on. OSR compilation still occurs in the main thread.
   ASSERT((osr_id != Thread::kNoDeoptId) || !FLAG_background_compilation ||
-         !thread->isolate()->MutatorThreadIsCurrentThread());
+         !thread->IsMutatorThread());
   CompilationPipeline* pipeline =
       CompilationPipeline::New(thread->zone(), function);
   return CompileFunctionHelper(pipeline, function, true, osr_id);