Implement part-of directive.

dart/lib/compiler/implementation/dart2js.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('dart2js');
 
 #import('dart:io');
 #import('dart:uri');
 #import('dart:utf');
 
@@ skipping 232 matching lines @@
     } else if (kind === api.Diagnostic.INFO) {
       color = colors.green;
     } else {
       throw 'Unknown kind: $kind (${kind.ordinal})';
     }
     if (uri === null) {
       assert(fatal);
       print(color(message));
     } else if (fatal || showWarnings) {
       SourceFile file = sourceFiles[uri.toString()];
+      if (file == null) {
+        throw '$uri: file is null';
+      }
       print(file.getLocationMessage(color(message), begin, end, true, color));
     }
     if (fatal && throwOnError) {
       isAborting = true;
       throw new AbortLeg(message);
     }
   }
 
   Uri uri = cwd.resolve(arguments[0]);
   if (packageRoot === null) {
@@ skipping 147 matching lines @@
     } catch (ignored) {
       print('Internal error: error while printing exception');
     }
     try {
       print(trace);
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   }
 }