Handle hiding dart:core imports specially.

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

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 186 matching lines @@
       "Error: Duplicate definition of '#{name}'.");
 
   static const MessageKind EXISTING_DEFINITION = const MessageKind(
       "Info: Existing definition of '#{name}'.");
 
   static const DualKind DUPLICATE_IMPORT = const DualKind(
       error: const MessageKind("Error: Duplicate import of '#{name}'."),
       warning: const MessageKind("Warning: Duplicate import of '#{name}'."));
 
   static const MessageKind HIDDEN_IMPORT = const MessageKind(
-      "Warning: '#{name}' from library '#{hiddenUri}' by '#{name}' "
+      "Warning: '#{name}' from library '#{hiddenUri}' is hidden by '#{name}' "
       "from library '#{hidingUri}'.",
       howToFix: "Try adding 'hide #{name}' to the import of '#{hiddenUri}'.",
       examples: const [
           const {
 'main.dart':
 """
 import 'dart:async'; // This imports a class Future.
 import 'future.dart';
 
 void main() {}""",
@@ skipping 31 matching lines @@
 library future;
 
 class Future {}""",
 
 'export.dart':
 """
 library export;
 
 export 'future.dart';"""}]);
 
+
+  static const MessageKind HIDDEN_IMPLICIT_IMPORT = const MessageKind(
+      "Warning: '#{name}' from library '#{hiddenUri}' is hidden by '#{name}' "
+      "from library '#{hidingUri}'.",
+      howToFix: "Try adding an explicit "
+                "'import \"#{hiddenUri}\" hide #{name}'.",
+      examples: const [
+          const {
+'main.dart':
+"""
+// This hides the implicit import of class Type from dart:core.
+import 'type.dart';
+
+void main() {}""",
+
+'type.dart':
+"""
+library type;
+
+class Type {}"""}]);
+
   static const MessageKind DUPLICATE_EXPORT = const MessageKind(
       "Error: Duplicate export of '#{name}'.");
 
   static const DualKind NOT_A_TYPE = const DualKind(
       error: const MessageKind("Error: '#{node}' is not a type."),
       warning: const MessageKind("Warning: '#{node}' is not a type."));
 
   static const MessageKind NOT_A_PREFIX = const MessageKind(
       "Error: '#{node}' is not a prefix.");
 
@@ skipping 1044 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }