Implement external strings.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index deac79d64b86369d519c52a5e2abc323d2406fc9..5316fdce9925c688ac88fce55ef215b37441a351 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -203,6 +203,12 @@ void ClassFinalizer::VerifyBootstrapClasses() {
   ASSERT(TwoByteString::InstanceSize() == cls.instance_size());
   cls = object_store->four_byte_string_class();
   ASSERT(FourByteString::InstanceSize() == cls.instance_size());
+  cls = object_store->external_one_byte_string_class();
+  ASSERT(ExternalOneByteString::InstanceSize() == cls.instance_size());
+  cls = object_store->external_two_byte_string_class();
+  ASSERT(ExternalTwoByteString::InstanceSize() == cls.instance_size());
+  cls = object_store->external_four_byte_string_class();
+  ASSERT(ExternalFourByteString::InstanceSize() == cls.instance_size());
   cls = object_store->double_class();
   ASSERT(Double::InstanceSize() == cls.instance_size());
   cls = object_store->mint_class();