Throw if there is no compiler context available

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

 // Copyright (c) 2017, 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_context;
 
 import 'dart:async' show Zone, runZoned;
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/physical_file_system.dart';
+import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:kernel/ast.dart' show Source;
 
 import 'compiler_command_line.dart' show CompilerCommandLine;
 
 import 'colors.dart' show computeEnableColors;
 
 import 'fasta_codes.dart' show LocatedMessage, Message;
 
 import 'severity.dart' show Severity;
 
 final Object compilerContextKey = new Object();
 
-final CompilerContext rootContext =
-    new CompilerContext(CompilerCommandLine.forRootContext());
-
 class CompilerContext {
   final FileSystem fileSystem = PhysicalFileSystem.instance;
 
   final CompilerCommandLine options;
 
   final Map<String, Source> uriToSource = <String, Source>{};
 
   bool enableColorsCached = null;
 
   CompilerContext(this.options);
@@ skipping 16 matching lines @@
   String format(LocatedMessage message, Severity severity) {
     return options.format(message, severity);
   }
 
   /// Format [message] as a text string that can be included in generated code.
   String formatWithoutLocation(Message message, Severity severity) {
     return options.formatWithoutLocation(message, severity);
   }
 
   static CompilerContext get current {
-    return Zone.current[compilerContextKey] ?? rootContext;
+    var context = Zone.current[compilerContextKey];
+    if (context == null) {
+      // Note: we throw directly and don't use internalProblem, because
+      // internalProblem depends on having a compiler context available.
+      var message = messageInternalProblemMissingContext.message;
+      var tip = messageInternalProblemMissingContext.tip;
+      throw "Internal problem: $message\nTip: $tip";
+    }
+    return context;
   }
 
   /// Perform [action] in a [Zone] where [cl] will be available as
   /// `CompilerContext.current.options`.
   static dynamic withGlobalOptions(
       CompilerCommandLine cl, dynamic action(CompilerContext c)) {
     CompilerContext c = new CompilerContext(cl);
     return runZoned(() => action(c), zoneValues: {compilerContextKey: c});
   }
 
   static bool get enableColors {
     return current.enableColorsCached ??= computeEnableColors(current);
   }
 }