Reland r24563 and r24564 with fixes cumbersome API leading to leaks.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 24817)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -379,18 +379,21 @@
 }
 
 
-void GCMarker::ProcessPeerReferents(PageSpace* page_space) {
-  PageSpace::PeerTable* peer_table = page_space->GetPeerTable();
-  PageSpace::PeerTable::iterator it = peer_table->begin();
-  while (it != peer_table->end()) {
-    RawObject* raw_obj = it->first;
-    ASSERT(raw_obj->IsHeapObject());
-    if (raw_obj->IsMarked()) {
-      // The object has survived.  Do nothing.
-      ++it;
-    } else {
-      // The object has become garbage.  Remove its record.
-      peer_table->erase(it++);
+void GCMarker::ProcessWeakTables(PageSpace* page_space) {
+  for (int sel = 0;
+       sel < Heap::kNumWeakSelectors;
+       sel++) {
+    WeakTable* table = heap_->GetWeakTable(
+        Heap::kOld, static_cast<Heap::WeakSelector>(sel));
+    intptr_t size = table->size();
+    for (intptr_t i = 0; i < size; i++) {
+      if (table->IsValidEntryAt(i)) {
+        RawObject* raw_obj = table->ObjectAt(i);
+        ASSERT(raw_obj->IsHeapObject());
+        if (!raw_obj->IsMarked()) {
+          table->InvalidateAt(i);
+        }
+      }
     }
   }
 }
@@ -408,7 +411,7 @@
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
   mark.Finalize();
-  ProcessPeerReferents(page_space);
+  ProcessWeakTables(page_space);
   Epilogue(isolate, invoke_api_callbacks);
 }