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

pkg/compiler/lib/src/common/backend_api.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.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show
@@ skipping 394 matching lines @@
 
   void registerAsyncMarker(FunctionElement element,
                            Enqueuer enqueuer,
                            Registry registry) {}
 
   /// Called when resolving a call to a foreign function.
   void registerForeignCall(Send node,
                            Element element,
                            CallStructure callStructure,
                            ForeignResolver resolver) {}
+
+  /// Returns the location of the patch-file associated with [libraryName]
+  /// resolved from [plaformConfigUri].
+  ///
+  /// Returns null if there is none.
+  Uri resolvePatchUri(String libraryName, Uri plaformConfigUri);
 }
 
 /// Interface for resolving calls to foreign functions.
 abstract class ForeignResolver {
   /// Returns the constant expression of [node], or `null` if [node] is not
   /// a constant expression.
   ConstantExpression getConstant(Node node);
 
   /// Registers [type] as instantiated.
   void registerInstantiatedType(InterfaceType type);
 
   /// Resolves [typeName] to a type in the context of [node].
   DartType resolveTypeFromString(Node node, String typeName);
 }
 
 /// Backend transformation methods for the world impacts.
 class ImpactTransformer {
   /// Transform the [ResolutionImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformResolutionImpact(ResolutionImpact worldImpact) {
     return worldImpact;
   }
 
   /// Transform the [CodegenImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformCodegenImpact(CodegenImpact worldImpact) {
     return worldImpact;
   }
 }