Handle implicitly hidden exports.

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

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 8b1c8a58f46dd59bd9b3ed2dedc791a413efcd03..b51f5aabe06456fa5de17039c254ce3e83fb0d45 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -320,6 +320,34 @@ main() {}""",
   static const MessageKind DUPLICATE_EXPORT_DECL = const MessageKind(
       "The exported '#{name}' from export #{uriString} is defined here.");
 
+  static const MessageKind HIDDEN_IMPLICIT_EXPORT = const MessageKind(
+      "'#{name}' from library '#{hiddenUri}' is hidden by '#{name}' "
+      "from library '#{hidingUri}'.",
+      howToFix: "Try adding an explicit "
+                "'export \"#{hiddenUri}\" hide #{name}'.",
+      examples: const [
+          const {
+'conflictsWithDartHtml.dart':
+"""
+typedef CanvasElement();
+""",
+
+'reexport.dart':
+r"""
+export 'dart:html';
+export 'conflictsWithDartHtml.dart';
+""",
+
+'main.dart':
+r"""
+import 'reexport.dart';
+
+main() {
+  print(main is CanvasElement);
+}
+"""}]);
+
+
   static const MessageKind NOT_A_TYPE = const MessageKind(
       "'#{node}' is not a type.");
 
@@ -1256,6 +1284,9 @@ main() => A.A = 1;
   static const MessageKind IMPORTED_HERE = const MessageKind(
       "'#{name}' is imported here.");
 
+  static const MessageKind EXPORTED_HERE = const MessageKind(
+      "'#{name}' from '#{uri}' is exported here.");
+
   static const MessageKind OVERRIDE_EQUALS_NOT_HASH_CODE = const MessageKind(
       "The class '#{class}' overrides 'operator==', "
       "but not 'get hashCode'.");