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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 3800f22b4ea2fd1163c2f0e29d95ded08e88df94..472c1f94672491816603d7b6bbd65f17ca3ec35d 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -273,7 +273,8 @@ class MockCompiler extends Compiler {
               analyzeAllFlag: analyzeAll,
               analyzeOnly: analyzeOnly,
               emitJavaScript: emitJavaScript,
-              preserveComments: preserveComments) {
+              preserveComments: preserveComments,
+              showPackageWarnings: true) {
     clearMessages();
     coreLibrary = createLibrary("core", coreSource);