Remove support for missing part-of tag.

sdk/lib/_internal/compiler/implementation/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.
 
 part of dart2js;
 
 /**
  * [CompilerTask] for loading libraries and setting up the import/export scopes.
  *
  * The library loader uses four different kinds of URIs in different parts of
@@ skipping 350 matching lines @@
    */
   void scanPart(Part part, Uri resolvedUri, LibraryElement library) {
     if (!resolvedUri.isAbsolute) throw new ArgumentError(resolvedUri);
     Uri readableUri = compiler.translateResolvedUri(library, resolvedUri, part);
     Script sourceScript = compiler.readScript(readableUri, part);
     CompilationUnitElement unit =
         new CompilationUnitElementX(sourceScript, library);
     compiler.withCurrentElement(unit, () {
       compiler.scanner.scan(unit);
       if (unit.partTag == null) {
-        bool wasDiagnosticEmitted = false;
-        compiler.withCurrentElement(library, () {
-          wasDiagnosticEmitted =
-              compiler.onDeprecatedFeature(part, 'missing part-of tag');
-        });
-        if (wasDiagnosticEmitted) {
-          compiler.reportMessage(
-              compiler.spanFromElement(unit),
-              MessageKind.MISSING_PART_OF_TAG.error(),
-              api.Diagnostic.INFO);
-        }
+        compiler.reportError(unit, MessageKind.MISSING_PART_OF_TAG);
       }
     });
   }
 
   /**
    * Handle an import/export tag by loading the referenced library and
    * registering its dependency in [handler] for the computation of the import/
    * export scope.
    */
   void registerLibraryFromTag(LibraryDependencyHandler handler,
@@ skipping 447 matching lines @@
   }
 
   /**
    * Registers all top-level entities of [library] as starting point for the
    * fixed-point computation of the import/export scopes.
    */
   void registerLibraryExports(LibraryElement library) {
     nodeMap[library].registerInitialExports();
   }
 }