dart2js: complain about missing part-of tags.

dart/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.
  */
 abstract class LibraryLoader extends CompilerTask {
@@ skipping 250 matching lines @@
 
   /**
    * Handle a part tag in the scope of [library]. The [path] given is used as
    * is, any URI resolution should be done beforehand.
    */
   void scanPart(Part part, Uri path, LibraryElement library) {
     if (!path.isAbsolute()) throw new ArgumentError(path);
     Script sourceScript = compiler.readScript(path, part);
     CompilationUnitElement unit =
         new CompilationUnitElement(sourceScript, library);
-    compiler.withCurrentElement(unit, () => compiler.scanner.scan(unit));
+    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);
+        }
+      }
+    });
   }
 
   /**
    * 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,
                               LibraryElement library,
                               LibraryDependency tag) {
@@ skipping 418 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();
   }
 }