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

pkg/compiler/lib/src/dart_backend/backend.dart

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index 5db667a7cec186d11f6641fc0822571e1a432102..de758c125cd7719db7dce457d550418a0bda0e60 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -355,6 +355,12 @@ class DartBackend extends Backend {
         node, MessageKind.DEFERRED_LIBRARY_DART_2_DART);
     return false;
   }
+
+  @override
+  Uri resolvePatchUri(String libraryName, Uri) {
+    // Dart2dart does not use patches.
+    return null;
+  }
 }
 
 class DartImpactTransformer extends ImpactTransformer {