Strings can be concatenated as follows:

  String s1 = "Dart", s2 = "Cook", s3 = "Book";
  var res = "Dart" " Cook" "Book";        (1)
  res = "Dart"  " Cook"
              "Book";        (2)
  res = s1 + " " + s2 + s3;        (3)
  res = "$s1 $s2$s3";        (4)
  res = [s1, " ", s2, s3].join();        (5)

  var sb = new StringBuffer();        (6)
  sb.writeAll([s1, " ", s2, s3]);
  res = sb.toString();
  print(res); // Dart CookBook

Adjacent string literals are taken together as one string as shown in line (1), even if they are on different lines as shown in line (2). The + operator does the same thing (3), as well as string interpolation (4), which is the preferred way. Still there is another way to add join() to List<String> as shown in line (5). The most efficient way, especially if you want...