Throw if there is no compiler context available

pkg/front_end/lib/src/fasta/compiler_command_line.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.compiler_command_line;
 
 import 'dart:io' show exit;
 
 import 'package:kernel/target/targets.dart'
     show Target, getTarget, TargetFlags, targets;
@@ skipping 138 matching lines @@
   String Function(Message, Severity) get formatWithoutLocation {
     return options["formatWithoutLocation"] ??
         command_line_reporting.formatWithoutLocation;
   }
 
   static dynamic withGlobalOptions(String programName, List<String> arguments,
       dynamic f(CompilerContext context)) {
     return CompilerContext.withGlobalOptions(
         new CompilerCommandLine(programName, arguments), f);
   }
-
-  static CompilerCommandLine forRootContext() {
-    return new CompilerCommandLine("", [""]);
-  }
 }
 
 Message computeUsage(String programName, bool verbose) {
   String basicUsage = "Usage: $programName [options] dartfile\n";
   String summary;
   String options =
       (verbose ? messageFastaUsageLong.message : messageFastaUsageShort.message)
           .trim();
   switch (programName) {
     case "outline":
@@ skipping 17 matching lines @@
   StringBuffer sb = new StringBuffer(basicUsage);
   if (summary != null) {
     sb.writeln();
     sb.writeln(summary);
     sb.writeln();
   }
   sb.write(options);
   // TODO(ahe): Don't use [templateUnspecified].
   return templateUnspecified.withArguments("$sb");
 }