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

tests/compiler/dart2js/mock_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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 255 matching lines @@
                 int this.expectedErrors})
       : sourceFiles = new Map<String, SourceFile>(),
         super(enableTypeAssertions: enableTypeAssertions,
               enableMinification: enableMinification,
               enableConcreteTypeInference: enableConcreteTypeInference,
               maxConcreteTypeSize: maxConcreteTypeSize,
               disableTypeInferenceFlag: disableTypeInference,
               analyzeAllFlag: analyzeAll,
               analyzeOnly: analyzeOnly,
               emitJavaScript: emitJavaScript,
-              preserveComments: preserveComments) {
+              preserveComments: preserveComments,
+              showPackageWarnings: true) {
     clearMessages();
     coreLibrary = createLibrary("core", coreSource);
 
     // We need to set the assert method to avoid calls with a 'null'
     // target being interpreted as a call to assert.
     jsHelperLibrary = createLibrary("helper", helperSource);
     foreignLibrary = createLibrary("foreign", FOREIGN_LIBRARY);
     interceptorsLibrary = createLibrary("interceptors", interceptorsSource);
     isolateHelperLibrary = createLibrary("isolate_helper", isolateHelperSource);
 
@@ skipping 287 matching lines @@
     } else {
       sourceFile = compiler.sourceFiles[uri.toString()];
     }
     if (sourceFile != null && begin != null && end != null) {
       print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
     } else {
       print(message);
     }
   };
 }