dart2js: throw when receiving an internal-problem error from the FE.

pkg/compiler/lib/src/kernel/front_end_adapter.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.
 
 /// Helper classes and methods to adapt between `package:compiler` and
 /// `package:front_end` APIs.
 library compiler.kernel.front_end_adapter;
 
 import 'dart:async';
 
@@ skipping 63 matching lines @@
   }
 }
 
 /// Report a [message] received from the front-end, using dart2js's
 /// [DiagnosticReporter].
 void reportFrontEndMessage(
     DiagnosticReporter reporter, fe.CompilationMessage message) {
   // TODO(sigmund): translate message kinds using message.dart2jsCode
   MessageKind kind = MessageKind.GENERIC;
   switch (message.severity) {
+    case fe.Severity.internalProblem:
+      throw message.message;
     case fe.Severity.error:
-    case fe.Severity.internalProblem:
       reporter.reportErrorMessage(
           NO_LOCATION_SPANNABLE, kind, {'text': message.message});
       break;
     case fe.Severity.warning:
       reporter.reportWarningMessage(
           NO_LOCATION_SPANNABLE, kind, {'text': message.message});
       break;
     case fe.Severity.nit:
       reporter.reportHintMessage(
           NO_LOCATION_SPANNABLE, kind, {'text': message.message});
       break;
     default:
       throw new UnimplementedError('unhandled severity ${message.severity}');
   }
 }