Implement external strings.

runtime/vm/object_store.cc

Index: runtime/vm/object_store.cc
diff --git a/runtime/vm/object_store.cc b/runtime/vm/object_store.cc
index a048e412b902640b584eeacd0a7e331c2358a331..d45303a32ed62056d7076297c0a7788b71eeb787 100644
--- a/runtime/vm/object_store.cc
+++ b/runtime/vm/object_store.cc
@@ -73,6 +73,9 @@ RawClass* ObjectStore::GetClass(int index) {
     case kOneByteStringClass: return one_byte_string_class_;
     case kTwoByteStringClass: return two_byte_string_class_;
     case kFourByteStringClass: return four_byte_string_class_;
+    case kExternalOneByteStringClass: return external_one_byte_string_class_;
+    case kExternalTwoByteStringClass: return external_two_byte_string_class_;
+    case kExternalFourByteStringClass: return external_four_byte_string_class_;
     case kBoolClass: return bool_class_;
     case kArrayClass: return array_class_;
     case kImmutableArrayClass: return immutable_array_class_;
@@ -104,6 +107,12 @@ int ObjectStore::GetClassIndex(const RawClass* raw_class) {
     return kTwoByteStringClass;
   } else if (raw_class == four_byte_string_class_) {
     return kFourByteStringClass;
+  } else if (raw_class == external_one_byte_string_class_) {
+    return kExternalOneByteStringClass;
+  } else if (raw_class == external_two_byte_string_class_) {
+    return kExternalTwoByteStringClass;
+  } else if (raw_class == external_four_byte_string_class_) {
+    return kExternalFourByteStringClass;
   } else if (raw_class == bool_class_) {
     return kBoolClass;
   } else if (raw_class == array_class_) {