Remove support for missing part-of tag.

tests/compiler/dart2js/deprecated_features_test.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.
 
 // Test that deprecated language features are diagnosed correctly.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
@@ skipping 36 matching lines @@
       // offset<source>:path:kind: message
       //
       // "offset" is the character offset from the beginning of TEST_SOURCE.
       // "source" is the substring of TEST_SOURCE that the compiler is
       // indicating as erroneous.
       // "path" is the URI path.
       // "kind" is the result of calling toString on a [Diagnostic] object.
       // "message" is the expected message as a [String].  This is a
       // short-term solution and should eventually changed to include
       // a symbolic reference to a MessageKind.
-      "15<part 'part.dart';>::${deprecatedMessage('missing part-of tag')}\n"
-      "0<>:/part.dart:info: Note: This file has no part-of tag, but it is being"
-      " used as a part.\n"
-      "53<()>::${deprecatedMessage('getter parameters')}\n",
+      "19<()>::${deprecatedMessage('getter parameters')}\n",
       messages.toString());
 }
 
 deprecatedMessage(feature) {
   return
     "warning: Warning: Deprecated language feature, $feature"
     ", will be removed in a future Dart milestone.";
 }
 
 const Map<String, String> TEST_SOURCE =
   const <String, String>{ '': """
-library test;
-
-part 'part.dart';
-
 class Foo {
   get x() => null;
 }
 
 main() {
   var a = new Foo();
 }
 """,
-    // TODO(ahe): Why isn't this 'part.dart'? Why the leading slash?
-    '/part.dart': '',
   };