Honor -Yno-imports flag

If the flag is set, no root imports are added.

Author: odersky

src/dotty/tools/dotc/Compiler.scala

@@ -103,9 +103,11 @@ class Compiler {
       .setMode(Mode.ImplicitsEnabled)
       .setTyperState(new MutableTyperState(ctx.typerState, new ConsoleReporter()(ctx), isCommittable = true))
     ctx.definitions.init(start) // set context of definitions to start
+    start.setRunInfo(new RunInfo(start))
     def addImport(ctx: Context, sym: Symbol) =
       ctx.fresh.setImportInfo(ImportInfo.rootImport(sym)(ctx))
-    (start.setRunInfo(new RunInfo(start)) /: defn.RootImports)(addImport)
+    if (ctx.settings.YnoImports.value) start
+    else (start /: defn.RootImports)(addImport)
   }
 
   def reset()(implicit ctx: Context): Unit = {

src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -121,7 +121,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val log = PhasesSetting("-Ylog", "Log operations during")
   val Ylogcp = BooleanSetting("-Ylog-classpath", "Output information about what classpath is being applied.")
   val Ynogenericsig = BooleanSetting("-Yno-generic-signatures", "Suppress generation of generic signatures for Java.")
-  val noimports = BooleanSetting("-Yno-imports", "Compile without importing scala.*, java.lang.*, or Predef.")
+  val YnoImports = BooleanSetting("-Yno-imports", "Compile without importing scala.*, java.lang.*, or Predef.")
   val nopredef = BooleanSetting("-Yno-predef", "Compile without importing Predef.")
   val noAdaptedArgs = BooleanSetting("-Yno-adapted-args", "Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.")
   val selfInAnnots = BooleanSetting("-Yself-in-annots", "Include a \"self\" identifier inside of annotations.")