supress hint when using "show dynamic". This is a temporary fix to hide a hint

pkg/compiler/lib/src/library_loader.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.library_loader;
 
 import 'dart:async';
 
 import 'common/names.dart' show Uris;
 import 'common/tasks.dart' show CompilerTask, Measurer;
@@ skipping 1317 matching lines @@
           if (combinator.isHide) {
             if (library.isPackageLibrary &&
                 reporter.options.hidePackageWarnings) {
               // Only report hide hint on packages if we show warnings on these:
               // The hide may be non-empty in some versions of the package, in
               // which case you shouldn't remove the combinator.
               continue;
             }
             reporter.reportHintMessage(identifier, MessageKind.EMPTY_HIDE,
                 {'uri': library.canonicalUri, 'name': name});
-          } else {
+          } else if (!library.isDartCore || name != 'dynamic') {
+            // TODO(sigmund): remove this condition, we don't report a hint for
+            // `import "dart:core" show dynamic;` until our tools match in
+            // semantics (see #29125).
             reporter.reportHintMessage(identifier, MessageKind.EMPTY_SHOW,
                 {'uri': library.canonicalUri, 'name': name});
           }
         }
       }
     }
   }
 }
 
 /**
@@ skipping 329 matching lines @@
 }
 
 /// API used by the library loader to synchronously scan a library or
 /// compilation unit and ensure that their library tags are computed.
 abstract class ElementScanner {
   void scanLibrary(LibraryElement library);
   void scanUnit(CompilationUnitElement unit);
 }
 
 const _reuseLibrarySubtaskName = "Reuse library";