Make check in GetExternalString a runtime check instead of ASSERT....

include/v8.h

 // Copyright 2007-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 809 matching lines @@
     virtual size_t length() const = 0;
    protected:
     ExternalAsciiStringResource() {}
    private:
     // Disallow copying and assigning.
     ExternalAsciiStringResource(const ExternalAsciiStringResource&);
     void operator=(const ExternalAsciiStringResource&);
   };
 
   /**
-   * Get the ExternalStringResource for an external string.  Only
-   * valid if IsExternal() returns true.
+   * Get the ExternalStringResource for an external string.  Returns
+   * NULL if IsExternal() doesn't return true.
    */
   ExternalStringResource* GetExternalStringResource() const;
 
   /**
    * Get the ExternalAsciiStringResource for an external ascii string.
-   * Only valid if IsExternalAscii() returns true.
+   * Returns NULL if IsExternalAscii() doesn't return true.
    */
   ExternalAsciiStringResource* GetExternalAsciiStringResource() const;
 
   static String* Cast(v8::Value* obj);
 
   /**
    * Allocates a new string from either utf-8 encoded or ascii data.
    * The second parameter 'length' gives the buffer length.
    * If the data is utf-8 encoded, the caller must
    * be careful to supply the length parameter.
@@ skipping 1744 matching lines @@
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef V8EXPORT_INLINE
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_