Fix error string to include information about class and library when the snapshot writer throws an …

runtime/vm/snapshot.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_SNAPSHOT_H_
 #define VM_SNAPSHOT_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
@@ skipping 578 matching lines @@
   void set_exception_msg(const char* msg) {
     exception_msg_ = msg;
   }
   void ThrowException(Exceptions::ExceptionType type, const char* msg);
 
   // Write a version string for the snapshot.
   void WriteVersion();
 
  protected:
   void UnmarkAll() {
-    forward_list_.UnmarkAll();
+    if (!unmarked_objects_) {
+      forward_list_.UnmarkAll();
+      unmarked_objects_ = true;
+    }
   }
 
   bool CheckAndWritePredefinedObject(RawObject* raw);
   void HandleVMIsolateObject(RawObject* raw);
 
   void WriteObjectRef(RawObject* raw);
   void WriteClassId(RawClass* cls);
   void WriteObjectImpl(RawObject* raw);
   void WriteInlinedObject(RawObject* raw);
   void WriteForwardedObjects();
@@ skipping 15 matching lines @@
   ObjectStore* object_store() const { return object_store_; }
 
  private:
   Snapshot::Kind kind_;
   Isolate* isolate_;
   ObjectStore* object_store_;  // Object store for common classes.
   ClassTable* class_table_;  // Class table for the class index to class lookup.
   ForwardList forward_list_;
   Exceptions::ExceptionType exception_type_;  // Exception type.
   const char* exception_msg_;  // Message associated with exception.
+  bool unmarked_objects_;  // True if marked objects have been unmarked.
 
   friend class RawArray;
   friend class RawClass;
   friend class RawClosureData;
   friend class RawGrowableObjectArray;
   friend class RawLinkedHashMap;
   friend class RawImmutableArray;
   friend class RawJSRegExp;
   friend class RawLibrary;
   friend class RawLiteralToken;
@@ skipping 82 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_