Don't output tons of messages on apidoc.

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

Index: sdk/lib/_internal/compiler/implementation/library_loader.dart
diff --git a/sdk/lib/_internal/compiler/implementation/library_loader.dart b/sdk/lib/_internal/compiler/implementation/library_loader.dart
index 216bba8f806aeae84cde15a6eceebfccdebb6680..46a8b34d4122d7a35e302fda37e3633e0244b7f8 100644
--- a/sdk/lib/_internal/compiler/implementation/library_loader.dart
+++ b/sdk/lib/_internal/compiler/implementation/library_loader.dart
@@ -320,12 +320,12 @@ class LibraryLoaderTask extends LibraryLoader {
         compiler.reportMessage(
             compiler.spanFromSpannable(tag.name, uri),
             MessageKind.DUPLICATED_LIBRARY_NAME.error({'libraryName': name}),
-            api.Diagnostic.WARNING);
+            api.Diagnostic.VERBOSE_INFO);
         Uri existingUri = existing.entryCompilationUnit.script.uri;
         compiler.reportMessage(
             compiler.spanFromSpannable(existing.libraryTag.name, existingUri),
             MessageKind.DUPLICATED_LIBRARY_NAME.error({'libraryName': name}),
-            api.Diagnostic.WARNING);
+            api.Diagnostic.VERBOSE_INFO);
       }
     }
   }