Introduce "platform configurations" to replace categories and libraries.dart.

pkg/compiler/samples/darttags/darttags.dart

 // Copyright (c) 2013, 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.
 
 // Usage: Add the following to your .gclient file (found in the parent
 // of the "dart" in a gclient checkout of the Dart repositor).
 //
 // hooks = [
 //   {
 //     "pattern": ".",
@@ skipping 69 matching lines @@
     uris.add(myLocation.resolve(DART2JS));
     uris.add(myLocation.resolve(DART2JS_MIRROR));
   }
 
   // Get the names of public dart2js libraries.
   Iterable<String> names = libraries.keys.where(isPublicDart2jsLibrary);
 
   // Prepend "dart:" to the names.
   uris.addAll(names.map((String name) => Uri.parse('dart:$name')));
 
-  Uri libraryRoot = myLocation.resolve(SDK_ROOT);
+  Uri platformConfigUri = myLocation.resolve(SDK_ROOT)
+      .resolve("lib/dart2js_shared_sdk");
   Uri packageRoot = Uri.base.resolve(Platform.packageRoot);
 
-  analyze(uris, libraryRoot, packageRoot, handler.provider, handler)
+  analyze(uris, platformConfigUri, packageRoot, handler.provider, handler)
       .then(processMirrors);
 }
 
 processMirrors(MirrorSystem mirrors) {
   mirrors.libraries.forEach((_, LibraryMirror library) {
     BackDoor.compilationUnitsOf(library).forEach(emitTagsForCompilationUnit);
   });
 
   output.closeSync();
 }
@@ skipping 82 matching lines @@
 
     return new Definition(byte_offset, line_number, tag_definition_text);
   }
 
   void writeOn(StringBuffer buffer, String tagname) {
     buffer.write(
         '${tag_definition_text}\x7f${tagname}'
         '\x01${line_number},${byte_offset}\n');
   }
 }