The StringBuffer class is useful for concatenating strings efficiently. Only on a call to toString are the strings concatenated to a single String.

class StringBuffer implements StringSink {

 /** Creates the string buffer with an initial content. */
 external StringBuffer([Object content = ""]);

 /**
  * Returns the length of the content that has been accumulated so far.
  * This is a constant-time operation.
  */
 external int get length;

 /** Returns whether the buffer is empty. This is a constant-time operation. */
 bool get isEmpty => length == 0;

 /**
  * Returns whether the buffer is not empty. This is a constant-time
  * operation.
  */
 bool get isNotEmpty => !isEmpty;

 /// Adds the contents of [obj], converted to a string, to the buffer.
 external void write(Object obj);

 /// Adds the string representation of [charCode] to the buffer.
 external void writeCharCode(int charCode);

 void writeAll(Iterable objects, [String separator = ""]) {
   Iterator iterator = objects.iterator;
   if (!iterator.moveNext()) return;
   if (separator.isEmpty) {
     do {
       write(iterator.current);
     } while (iterator.moveNext());
   } else {
     write(iterator.current);
     while (iterator.moveNext()) {
       write(separator);
       write(iterator.current);
     }
   }
 }

 void writeln([Object obj = ""]) {
   write(obj);
   write("\n");
 }

 /**
  * Clears the string buffer.
  */
 external void clear();

 /// Returns the contents of buffer as a concatenated string.
 external String toString();
}