Compilation with JDK 11 failed due to changes in the Java String API

[Glavo]

The String#lines method has been added to the Java 11 String, this change causes Dotty to fail to compile:

[error] /home/glavo/Dotty/dotty/compiler/src/dotty/tools/dotc/util/ParsedComment.scala:151:44: value mkString is not a member of java.util.stream.Stream[String]
[error]     val formattedItems = items.map(_.lines.mkString(System.lineSeparator + "   "))
[error]                                            ^
[error] one error found
[error] (dotty-compiler / Compile / compileIncremental) Compilation failed
[error] Total time: 29 s, completed 2018年11月19日 下午5:35:12