Moves the top_ and end_ words of the Scavenger into mutator thread.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 2309e6ca2aa3bb5c5b08cdf062cc4556206060b5..10b07c8ed9cf76c149a8b43fdff1c37e473e804e 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -337,7 +337,8 @@ Scavenger::Scavenger(Heap* heap,
       delayed_weak_properties_(NULL),
       gc_time_micros_(0),
       collections_(0),
-      external_size_(0) {
+      external_size_(0),
+      space_lock_(new Mutex()) {
   // Verify assumptions about the first word in objects which the scavenger is
   // going to use for forwarding pointers.
   ASSERT(Object::tags_offset() == 0);
@@ -365,6 +366,7 @@ Scavenger::Scavenger(Heap* heap,
 Scavenger::~Scavenger() {
   ASSERT(!scavenging_);
   to_->Delete();
+  delete space_lock_;
 }
 
 
@@ -387,6 +389,9 @@ SemiSpace* Scavenger::Prologue(Isolate* isolate, bool invoke_api_callbacks) {
     (isolate->gc_prologue_callback())();
   }
   isolate->PrepareForGC();
+
+  ASSERT(isolate->mutator_thread()->top() == top_);
+
   // Flip the two semi-spaces so that to_ is always the space for allocating
   // objects.
   SemiSpace* from = to_;
@@ -400,6 +405,10 @@ SemiSpace* Scavenger::Prologue(Isolate* isolate, bool invoke_api_callbacks) {
   top_ = FirstObjectStart();
   resolved_top_ = top_;
   end_ = to_->end();
+
+  isolate->mutator_thread()->set_top_offset(top_);
+  isolate->mutator_thread()->set_end_offset(end_);
+
   return from;
 }
 
@@ -409,6 +418,10 @@ void Scavenger::Epilogue(Isolate* isolate,
                          bool invoke_api_callbacks) {
   // All objects in the to space have been copied from the from space at this
   // moment.
+
+  isolate->mutator_thread()->set_top_offset(top_);
+  isolate->mutator_thread()->set_end_offset(end_);
+
   double avg_frac = stats_history_.Get(0).PromoCandidatesSuccessFraction();
   if (stats_history_.Size() >= 2) {
     // Previous scavenge is only given half as much weight.
@@ -545,6 +558,9 @@ void Scavenger::IterateWeakRoots(Isolate* isolate, HandleVisitor* visitor) {
 
 void Scavenger::ProcessToSpace(ScavengerVisitor* visitor) {
   // Iterate until all work has been drained.
+
+  ASSERT(Isolate::Current()->mutator_thread()->top() == top_);
+
   while ((resolved_top_ < top_) || PromotedStackHasMore()) {
     while (resolved_top_ < top_) {
       RawObject* raw_obj = RawObject::FromAddr(resolved_top_);