Document that lines and columns start at 0.

And adjust for it in DottyBackendInterface

Author: odersky

src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -480,7 +480,7 @@ class DottyBackendInterface()(implicit ctx: Context) extends BackendInterface{
 
   implicit def positionHelper(a: Position): PositionHelper = new PositionHelper {
     def isDefined: Boolean = a.exists
-    def line: Int = sourcePos(a).line
+    def line: Int = sourcePos(a).line + 1
     def finalPosition: Position = a
   }
 

src/dotty/tools/dotc/util/SourcePosition.scala

@@ -10,7 +10,11 @@ case class SourcePosition(source: SourceFile, pos: Position) {
   def exists = pos.exists
 
   def lineContents: String = source.lineContents(point)
+
+  /** The line of the position, starting at 0 */
   def line: Int = source.offsetToLine(point)
+
+  /** The column of the position, starting at 0 */
   def column: Int = source.column(point)
 
   override def toString =