Implement external strings.

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
diff --git a/runtime/vm/object_store.h b/runtime/vm/object_store.h
index 99539fd8258c80c702b3244bc73e2eb663c97638..cd1eb6991aff60670308de0095f24ca4764c509b 100644
--- a/runtime/vm/object_store.h
+++ b/runtime/vm/object_store.h
@@ -42,6 +42,9 @@ class ObjectStore {
     kOneByteStringClass,
     kTwoByteStringClass,
     kFourByteStringClass,
+    kExternalOneByteStringClass,
+    kExternalTwoByteStringClass,
+    kExternalFourByteStringClass,
     kBoolClass,
     kArrayClass,
     kImmutableArrayClass,
@@ -125,6 +128,27 @@ class ObjectStore {
     four_byte_string_class_ = value.raw();
   }
 
+  RawClass* external_one_byte_string_class() const {
+    return external_one_byte_string_class_;
+  }
+  void set_external_one_byte_string_class(const Class& value) {
+    external_one_byte_string_class_ = value.raw();
+  }
+
+  RawClass* external_two_byte_string_class() const {
+    return external_two_byte_string_class_;
+  }
+  void set_external_two_byte_string_class(const Class& value) {
+    external_two_byte_string_class_ = value.raw();
+  }
+
+  RawClass* external_four_byte_string_class() const {
+    return external_four_byte_string_class_;
+  }
+  void set_external_four_byte_string_class(const Class& value) {
+    external_four_byte_string_class_ = value.raw();
+  }
+
   RawType* bool_interface() const { return bool_interface_; }
   void set_bool_interface(const Type& value) { bool_interface_ = value.raw(); }
 
@@ -253,6 +277,9 @@ class ObjectStore {
   RawClass* one_byte_string_class_;
   RawClass* two_byte_string_class_;
   RawClass* four_byte_string_class_;
+  RawClass* external_one_byte_string_class_;
+  RawClass* external_two_byte_string_class_;
+  RawClass* external_four_byte_string_class_;
   RawType* bool_interface_;
   RawClass* bool_class_;
   RawClass* array_class_;