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

runtime/vm/object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 6505 matching lines @@
                                             Snapshot::Kind kind);
 
   static intptr_t NextFieldOffset() {
     // Indicates this class cannot be extended by dart code.
     return -kWordSize;
   }
 
   friend class Class;
   friend class String;
   friend class SnapshotReader;
+  friend class Symbols;
 };
 
 
 class ExternalTwoByteString : public AllStatic {
  public:
   static uint16_t CharAt(const String& str, intptr_t index) {
     NoSafepointScope no_safepoint;
     return *CharAddr(str, index);
   }
 
@@ skipping 58 matching lines @@
                                             Snapshot::Kind kind);
 
   static intptr_t NextFieldOffset() {
     // Indicates this class cannot be extended by dart code.
     return -kWordSize;
   }
 
   friend class Class;
   friend class String;
   friend class SnapshotReader;
+  friend class Symbols;
 };
 
 
 // Class Bool implements Dart core class bool.
 class Bool : public Instance {
  public:
   bool value() const {
     return raw_ptr()->value_;
   }
 
@@ skipping 1447 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_