Initial support for length guards on final fields.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 73afd054c7fa37a863986fbb9305d8fbce35183f..027fa176ee2715021c8f3d6066a149e8bbb341e7 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1982,11 +1982,28 @@ class Field : public Object {
   static intptr_t guarded_cid_offset() {
     return OFFSET_OF(RawField, guarded_cid_);
   }
+  // Return the list length that any list stored in this field is guaranteed
+  // to have. If length is kUnknownLength the length has not been determined.
+  // If length is kVariableLength this field has multiple list lengths
+  // associated with it and cannot be predicted.
+  intptr_t guarded_list_length() const {
+    return raw_ptr()->guarded_list_length_;
+  }
+  void set_guarded_list_length(intptr_t list_length) const {
+    raw_ptr()->guarded_list_length_ = list_length;
+  }
+  static intptr_t guarded_list_length_offset() {
+    return OFFSET_OF(RawField, guarded_list_length_);
+  }
 
   static bool IsExternalizableCid(intptr_t cid) {
     return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
   }
 
+  enum {
+    kUnknownLength = -1,
+    kNoLength = -2,
+  };
   // Returns false if any value read from this field is guaranteed to be
   // not null.
   // Internally we is_nullable_ field contains either kNullCid (nullable) or
@@ -2002,9 +2019,10 @@ class Field : public Object {
     return OFFSET_OF(RawField, is_nullable_);
   }
 
-  // Update guarded class id and nullability of the field to reflect assignment
-  // of the value with the given class id to this field.
+  // Update guarded class id, nullability, to reflect assignment of the
+  // value with the given class id to this field.
   void UpdateCid(intptr_t cid) const;
+  void UpdateLength(intptr_t length) const;
 
   // Return the list of optimized code objects that were optimized under
   // assumptions about guarded class id and nullability of this field.