Repl crashes when completing EmptyTree

[allanrenucci]

> repl
...
scala> #<tab>Exception in thread "main" java.lang.AssertionError: assertion failed
	at scala.Predef$.assert(Predef.scala:204)
	at dotty.tools.dotc.util.Positions$Position$.start$extension(Positions.scala:40)
	at dotty.tools.repl.ReplCompiler.wrap$1(ReplCompiler.scala:241)
	at dotty.tools.repl.ReplCompiler.wrapped$1(ReplCompiler.scala:249)
	at dotty.tools.repl.ReplCompiler.typeCheck(ReplCompiler.scala:284)
	at dotty.tools.repl.ReplDriver.completions(ReplDriver.scala:158)
	at dotty.tools.repl.ReplDriver.$anonfun$readLine$1(ReplDriver.scala:182)
	at dotty.tools.repl.ReplDriver.$anonfun$readLine$1$adapted(ReplDriver.scala:182)
	at dotty.tools.repl.AmmoniteReader.$anonfun$prompt$2(AmmoniteReader.scala:70)
	at dotty.tools.repl.terminal.Filter$$anon$3$$anonfun$op$8.applyOrElse(Filter.scala:66)
	at dotty.tools.repl.terminal.Filter$$anon$3$$anonfun$op$8.applyOrElse(Filter.scala:66)
	at scala.collection.TraversableOnce.collectFirst(TraversableOnce.scala:145)
	at scala.collection.TraversableOnce.collectFirst$(TraversableOnce.scala:132)
	at scala.collection.AbstractIterator.collectFirst(Iterator.scala:1417)
	at dotty.tools.repl.terminal.Filter$$anon$3.op(Filter.scala:66)
	at dotty.tools.repl.terminal.Filter$$anon$1.$anonfun$op$6(Filter.scala:88)
	at scala.collection.Iterator$$anon$10.next(Iterator.scala:448)
	at scala.collection.Iterator.find(Iterator.scala:980)
	at scala.collection.Iterator.find$(Iterator.scala:978)
	at scala.collection.AbstractIterator.find(Iterator.scala:1417)
	at dotty.tools.repl.terminal.Filter$$anon$1.op(Filter.scala:88)
	at dotty.tools.repl.terminal.Terminal$.readChar$1(Terminal.scala:273)
	at dotty.tools.repl.terminal.Terminal$.readLine(Terminal.scala:304)
	at dotty.tools.repl.AmmoniteReader.prompt(AmmoniteReader.scala:125)
	at dotty.tools.repl.ReplDriver.readLine(ReplDriver.scala:182)
	at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:133)
	at dotty.tools.repl.Main$.main(Main.scala:6)
	at dotty.tools.repl.Main.main(Main.scala)
java.lang.RuntimeException: Nonzero exit code returned from runner: 1
	at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last dotty-compiler/compile:runMain for the full output.
[error] (dotty-compiler/compile:runMain) Nonzero exit code returned from runner: 1
[error] Total time: 8 s, completed Oct 11, 2017 5:26:25 PM

The repl parses # which returns an empty tree. It then tries to access its position