Fix Dartium by supporting external strings as well in Symbols::FromConcatAll

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 04a8a0d6d8aa70d903f189a4352222e608170b99..331953f362352579854f4db08ac261410f3d9c4c 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6523,6 +6523,7 @@ class ExternalOneByteString : public AllStatic {
   friend class Class;
   friend class String;
   friend class SnapshotReader;
+  friend class Symbols;
 };
 
 
@@ -6601,6 +6602,7 @@ class ExternalTwoByteString : public AllStatic {
   friend class Class;
   friend class String;
   friend class SnapshotReader;
+  friend class Symbols;
 };