Load patches via callback.

sdk/lib/_internal/compiler/implementation/use_unused_api.dart

Index: sdk/lib/_internal/compiler/implementation/use_unused_api.dart
diff --git a/sdk/lib/_internal/compiler/implementation/use_unused_api.dart b/sdk/lib/_internal/compiler/implementation/use_unused_api.dart
index a19dae4566c99116912e35ebe6ee0f9b997fd58e..071126bc240815e60321665396048bcc0d6f7acd 100644
--- a/sdk/lib/_internal/compiler/implementation/use_unused_api.dart
+++ b/sdk/lib/_internal/compiler/implementation/use_unused_api.dart
@@ -67,6 +67,7 @@ void main(List<String> arguments) {
   usedByTests();
   useElements(null, null);
   useIr(null, null);
+  useCompiler(null);
 }
 
 useApi() {
@@ -231,3 +232,8 @@ useIr(ir_nodes.SExpressionStringifier stringifier,
     ..hasIr(null)
     ..getIr(null);
 }
+
+useCompiler(dart2jslib.Compiler compiler) {
+  compiler.libraryLoader.reset();
+  compiler.libraryLoader.reuseLibrary(null);
+}