Recover from fatal parser errors.

dart/pkg/compiler/lib/src/scanner/scanner_task.dart

 // Copyright (c) 2012, 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.
 
 part of scanner;
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name => 'Scanner';
 
@@ skipping 43 matching lines @@
 class DietParserTask extends CompilerTask {
   DietParserTask(Compiler compiler) : super(compiler);
   final String name = 'Diet Parser';
 
   dietParse(CompilationUnitElement compilationUnit, Token tokens) {
     measure(() {
       Function idGenerator = compiler.getNextFreeClassId;
       ElementListener listener =
           new ElementListener(compiler, compilationUnit, idGenerator);
       PartialParser parser = new PartialParser(listener);
-      parser.parseUnit(tokens);
+      try {
+        parser.parseUnit(tokens);
+      } on ParserError catch (e) {
+        assert(invariant(compilationUnit, compiler.compilationFailed));
+      }
     });
   }
 }