Move CompilerStats from isolate to thread. Aggregate stats.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 58ca52c434d7be7e3c21b0dbf0f00d5bc02da02f..9f4cec8f75c35791dba14c8781cffb6f1d3a39eb 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -418,8 +418,10 @@ class Isolate : public BaseIsolate {
 
   void PrintJSON(JSONStream* stream, bool ref = true);
 
-  CompilerStats* compiler_stats() {
-    return compiler_stats_;
+  // Mutator thread is used to aggregate compiler stats.
+  CompilerStats* aggregate_compiler_stats() {
+    ASSERT(HasMutatorThread());
+    return mutator_thread_->compiler_stats();
   }
 
   VMTagCounters* vm_tag_counters() {
@@ -671,8 +673,6 @@ class Isolate : public BaseIsolate {
   intptr_t defer_finalization_count_;
   DeoptContext* deopt_context_;
 
-  CompilerStats* compiler_stats_;
-
   bool is_service_isolate_;
 
   // Isolate-specific flags.