Provide a logical view of VM-internal maps in Observatory.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 39a57ba80d8ff26963d8dbc03075a943054417e9..a93b1814f0b775ed1e32636a286d8e265926bc13 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -7233,6 +7233,52 @@ class LinkedHashMap : public Instance {
     return OFFSET_OF(RawLinkedHashMap, deleted_keys_);
   }
 
+  intptr_t Length() const {
+    intptr_t used = Smi::Value(raw_ptr()->used_data_);
+    intptr_t deleted = Smi::Value(raw_ptr()->deleted_keys_);
+    return (used >> 1) - deleted;
+  }
+
+  class Iterator : ValueObject {

[koda, 2015/06/02 16:42:37]

Add comment to note how this differs from its Dart counterpart:
1. does not check for concurrent modifications.
2. crashes, rather than returns null, when the iterator is before beginning or
after end

[rmacnak, 2015/06/02 17:33:31]

Done.

+   public:
+    explicit Iterator(const LinkedHashMap& map)
+      : map_(map)
+      , data_(Array::Handle(map.data()))

[koda, 2015/06/02 16:42:37]

I think our code usually has these commas at the end of the lines.

[rmacnak, 2015/06/02 17:33:31]

Ah, yes. Before is Blink.

+      , scratch_(Object::Handle())
+      , offset_(-2)
+      , length_(Smi::Handle(map.used_data()).Value()) {}

[koda, 2015/06/02 16:42:37]

Avoid handle creation: Smi::Value(map.used_data())

[rmacnak, 2015/06/02 17:33:31]

Done.

+
+    bool MoveNext() {
+      // Cf. _CompactIterator in runtime/lib/compact_hash.dart.
+      while (true) {
+        offset_ += 2;
+        if (offset_ >= length_) {
+          return false;
+        }
+        scratch_ = data_.At(offset_);
+        if (scratch_.raw() != data_.raw()) {
+          // Slot is not deleted (self-reference indicates deletion).
+          return true;
+        }
+      }
+    }
+
+    RawObject* CurrentKey() const {
+      return data_.At(offset_);
+    }
+
+    RawObject* CurrentValue() const {
+      return data_.At(offset_ + 1);
+    }
+
+   private:
+    const LinkedHashMap& map_;
+    const Array& data_;
+    Object& scratch_;
+    intptr_t offset_;
+    const intptr_t length_;
+  };
+
  private:
   FINAL_HEAP_OBJECT_IMPLEMENTATION(LinkedHashMap, Instance);