Add a weak property type to the virtual machine.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 111fa03bc0708a09b6a200cf3befcd016741fec6..efe6c5778852db2abe2cdb441d38795dec6c3bb1 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -88,6 +88,7 @@ namespace dart {
       V(ExternalFloat64Array)                                                  \
     V(Stacktrace)                                                              \
     V(JSRegExp)                                                                \
+    V(WeakProperty)                                                            \
     V(Closure)                                                                 \
 
 #define CLASS_LIST(V)                                                          \
@@ -189,8 +190,9 @@ class RawObject {
     kMarkBit = 1,
     kCanonicalBit = 2,
     kFromSnapshotBit = 3,
-    kReservedTagBit = 4,  // kReservedBit{10K,100K,1M,10M}
-    kReservedTagSize = 4,
+    kWatchedBit = 4,
+    kReservedTagBit = 5,  // kReservedBit{10K,100K,1M,10M}
+    kReservedTagSize = 3,
     kSizeTagBit = 8,
     kSizeTagSize = 8,
     kClassIdTagBit = kSizeTagBit + kSizeTagSize,
@@ -261,6 +263,21 @@ class RawObject {
     ptr()->tags_ = MarkBit::update(false, tags);
   }
 
+  // Support for GC watched bit.
+  bool IsWatched() const {
+    return WatchedBit::decode(ptr()->tags_);
+  }
+  void SetWatchedBit() {
+    ASSERT(!IsWatched());
+    uword tags = ptr()->tags_;
+    ptr()->tags_ = WatchedBit::update(true, tags);
+  }
+  void ClearWatchedBit() {
+    ASSERT(IsWatched());
+    uword tags = ptr()->tags_;
+    ptr()->tags_ = WatchedBit::update(false, tags);
+  }
+
   // Support for object tags.
   bool IsCanonical() const {
     return CanonicalObjectTag::decode(ptr()->tags_);
@@ -330,6 +347,8 @@ class RawObject {
 
   class MarkBit : public BitField<bool, kMarkBit, 1> {};
 
+  class WatchedBit : public BitField<bool, kWatchedBit, 1> {};
+
   class CanonicalObjectTag : public BitField<bool, kCanonicalBit, 1> {};
 
   class CreatedFromSnapshotTag : public BitField<bool, kFromSnapshotBit, 1> {};
@@ -354,6 +373,7 @@ class RawObject {
   friend class Api;
   friend class Array;
   friend class FreeListElement;
+  friend class GCMarker;
   friend class Heap;
   friend class HeapProfiler;
   friend class HeapProfilerRootVisitor;
@@ -361,6 +381,7 @@ class RawObject {
   friend class Object;
   friend class RawInstructions;
   friend class RawInstance;
+  friend class Scavenger;
   friend class SnapshotReader;
   friend class SnapshotWriter;
 
@@ -1417,6 +1438,24 @@ class RawJSRegExp : public RawInstance {
   uint8_t data_[0];
 };
 
+class RawWeakProperty : public RawInstance {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(WeakProperty);
+
+  RawObject** from() {
+    return reinterpret_cast<RawObject**>(&ptr()->key_);
+  }
+  RawObject* key_;
+  RawObject* value_;
+  RawObject** to() {
+    return reinterpret_cast<RawObject**>(&ptr()->value_);
+  }
+
+  friend class GCMarker;
+  friend class MarkingVisitor;
+  friend class Scavenger;
+  friend class ScavengerVisitor;
+};
+
 // Class Id predicates.
 
 inline bool RawObject::IsErrorClassId(intptr_t index) {