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

tests/utils/dummy_compiler_test.dart

Index: tests/utils/dummy_compiler_test.dart
diff --git a/tests/utils/dummy_compiler_test.dart b/tests/utils/dummy_compiler_test.dart
index 43a79c58c51c70b3a3868953123a757f0f5c2441..608e0996686473dd6509b3daaeb00c21423f6f5f 100644
--- a/tests/utils/dummy_compiler_test.dart
+++ b/tests/utils/dummy_compiler_test.dart
@@ -15,7 +15,9 @@ import 'package:compiler/compiler.dart';
 import '../compiler/dart2js/mock_libraries.dart';
 
 String libProvider(Uri uri) {
-  if (uri.path.endsWith("/core.dart")) {
+  if (uri.path.endsWith(".platform")) {
+    return DEFAULT_PLATFORM_CONFIG;
+  } if (uri.path.endsWith("/core.dart")) {
     return buildLibrarySource(DEFAULT_CORE_LIBRARY);
   } else if (uri.path.endsWith('core_patch.dart')) {
     return DEFAULT_PATCH_CORE_SOURCE;