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

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index bdfcf79d0edd66ad46c7ed48ec8c17d0e0e2344f..04caaf8f47c7285621e5818a4fa619930fafa563 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -412,6 +412,12 @@ abstract class Backend {
                            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.