Don't crash when a file is missing.

dart/lib/compiler/implementation/apiimpl.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.
 
 #library('leg_apiimpl');
 
 #import('dart:uri');
 
 #import('../compiler.dart', prefix: 'api');
 #import('leg.dart', prefix: 'leg');
@@ skipping 36 matching lines @@
     var translated = translateUri(uri, node);
     String text = "";
     try {
       // TODO(ahe): We expect the future to be complete and call value
       // directly. In effect, we don't support truly asynchronous API.
       text = provider(translated).value;
     } catch (var exception) {
       if (node !== null) {
         cancel("$exception", node: node);
       } else {
-        reportDiagnostic(const leg.SourceSpan(null, null, null),
-                         "$exception", api.Diagnostic.ERROR);
+        reportDiagnostic(null, "$exception", api.Diagnostic.ERROR);
         throw new leg.CompilerCancelledException("$exception");
       }
     }
     SourceFile sourceFile = new SourceFile(uri.toString(), text);
     return new leg.Script(uri, sourceFile);
   }
 
   Uri translateUri(Uri uri, tree.Node node) {
     switch (uri.scheme) {
       case 'package': return translatePackageUri(uri, node);
@@ skipping 34 matching lines @@
       handler(translateUri(span.uri, null), span.begin, span.end,
               message, kind);
     }
   }
 
   bool get isMockCompilation() {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') !== -1);
   }
 }