Ignore StaleSymbol errors in IDE

Seems to happen in several scenarios. For now, ignore, so that we
can get real work done in the IDE. Ignoring is configurable,
we can turn it off if we want to hunt down problems.

Author: odersky

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -90,4 +90,7 @@ object Mode {
    *  In this case, identifiers should never be imported.
    */
   val InPackageClauseName = newMode(18, "InPackageClauseName")
+
+  /** We are in the IDE */
+  val Interactive = newMode(20, "Interactive")
 }

compiler/src/dotty/tools/dotc/interactive/InteractiveCompiler.scala

@@ -5,6 +5,7 @@ package interactive
 import core._
 import Phases._
 import typer._
+import Contexts._
 
 class InteractiveCompiler extends Compiler {
   // TODO: Figure out what phases should be run in IDEs
@@ -14,4 +15,7 @@ class InteractiveCompiler extends Compiler {
   override def phases: List[List[Phase]] = List(
     List(new FrontEnd)
   )
+
+  override def rootContext(implicit ctx: Context) =
+    super.rootContext.fresh.addMode(Mode.Interactive)
 }