Check java version at start.

Author: DarkDimius

compiler/src/dotty/tools/dotc/config/CompilerCommand.scala

@@ -115,6 +115,9 @@ object CompilerCommand extends DotClass {
     else if (settings.version.value) {
       ctx.echo(versionMsg)
       Nil
+    } else if (!Properties.isJavaAtLeast("1.8")) {
+      ctx.error("Dotty requires Java 8 or newer to run")
+      Nil
     }
     else if (shouldStopWithInfo) {
       ctx.echo(infoMessage)