Handle hiding dart:core imports specially.

sdk/lib/_internal/compiler/implementation/elements/modelx.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 elements.modelx;
 
 import 'dart:collection' show LinkedHashMap;
 
 import 'elements.dart';
 import '../../compiler.dart' as api;
@@ skipping 659 matching lines @@
    * [ErroneousElement] will be put in the imported scope, allowing for the
    * detection of ambiguous uses of imported names.
    */
   void addImport(Element element, Import import, DiagnosticListener listener) {
     importers[element] =
         importers.putIfAbsent(element, () => const Link<Import>())
         .prepend(import);
     SourceString name = element.name;
     Element existing = importScope.putIfAbsent(name, () => element);
     if (existing != element) {
+      // TODO(johnniwinther): Only emit these warnings if [element] is used.
       if (existing.getLibrary().isPlatformLibrary &&
           !element.getLibrary().isPlatformLibrary) {
         // [existing] is implicitly hidden.
         importScope[name] = element;
         listener.reportWarningCode(import, MessageKind.HIDDEN_IMPORT,
             {'name': name,
              'hiddenUri': existing.getLibrary().canonicalUri,
              'hidingUri': element.getLibrary().canonicalUri});
       } else if (!existing.getLibrary().isPlatformLibrary &&
                  element.getLibrary().isPlatformLibrary) {
         // [element] is implicitly hidden.
-        listener.reportWarningCode(import, MessageKind.HIDDEN_IMPORT,
-            {'name': name,
-             'hiddenUri': element.getLibrary().canonicalUri,
-             'hidingUri': existing.getLibrary().canonicalUri});
+        if (import == null) {
+          // [element] is imported implicitly (probably through dart:core).
+          listener.reportWarningCode(importers[existing].head,
+              MessageKind.HIDDEN_IMPLICIT_IMPORT,
+              {'name': name,
+               'hiddenUri': element.getLibrary().canonicalUri,
+               'hidingUri': existing.getLibrary().canonicalUri});
+        } else {
+          listener.reportWarningCode(import, MessageKind.HIDDEN_IMPORT,
+              {'name': name,
+               'hiddenUri': element.getLibrary().canonicalUri,
+               'hidingUri': existing.getLibrary().canonicalUri});
+        }
       } else {
         // TODO(johnniwinther): Provide access to the import tags from which
         // the elements came.
         importScope[name] = new AmbiguousElementX(
             MessageKind.DUPLICATE_IMPORT, {'name': name},
             this, existing, element);
       }
     }
   }
 
@@ skipping 1517 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }