Change --hide-package-warnings to --show-package-warnings and add hint on suppress warnings.

sdk/lib/_internal/compiler/implementation/source_file_provider.dart

 // Copyright (c) 2013, 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 source_file_provider;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 133 matching lines @@
       if (lastKind == api.Diagnostic.WARNING && !showWarnings) return;
       if (lastKind == api.Diagnostic.HINT && !showHints) return;
       color = colors.green;
     } else {
       throw 'Unknown kind: $kind (${kind.ordinal})';
     }
     if (!enableColors) {
       color = (x) => x;
     }
     if (uri == null) {
-      assert(fatal);
-      print(color(message));
+      print('${color(message)}');
     } else {
       SourceFile file = provider.sourceFiles[uri.toString()];
-      if (file == null) {
+      if (file != null) {
+        print(file.getLocationMessage(color(message), begin, end, true, color));
+      } else {
         throw '$uri: file is null';
       }
-      print(file.getLocationMessage(color(message), begin, end, true, color));
     }
     if (fatal && throwOnError) {
       isAborting = true;
       throw new AbortLeg(message);
     }
   }
 
   void call(Uri uri, int begin, int end, String message, api.Diagnostic kind) {
     return diagnosticHandler(uri, begin, end, message, kind);
   }
 }