Don't crash when reporting an error

pkg/compiler/lib/src/compiler.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 dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 1938 matching lines @@
       // position. So we use the enclosing element instead. It is
       // not a good error location, but cancel really is "internal
       // error" or "not implemented yet", so the vicinity is good
       // enough for now.
       element = element.enclosingElement;
       // TODO(ahe): I plan to overhaul this infrastructure anyways.
     }
     if (element == null) {
       element = currentElement;
     }
+    if (element == null) {
+      return null;
+    }
+
     if (element.sourcePosition != null) {
       return element.sourcePosition;
     }
     Token position = element.position;
     Uri uri = element.compilationUnit.script.readableUri;
     return (position == null)
         ? new SourceSpan(uri, 0, 0)
         : spanFromTokens(position, position, uri);
   }
 
@@ skipping 504 matching lines @@
     return futureClass.computeType(compiler).createInstantiation([elementType]);
   }
 
   @override
   InterfaceType streamType([DartType elementType = const DynamicType()]) {
     return streamClass.computeType(compiler).createInstantiation([elementType]);
   }
 }
 
 typedef void InternalErrorFunction(Spannable location, String message);