Read SDK and patches from a JSON file.

pkg/front_end/lib/src/fasta/dill/dill_target.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_target.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_target.dart b/pkg/front_end/lib/src/fasta/dill/dill_target.dart
index 858264fc7fbdd77642446d0b681e773122c1ac01..c628e52b2f264bc12876a15fa5b77cfa5bb2fb71 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_target.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_target.dart
@@ -50,7 +50,8 @@ class DillTarget extends TargetImplementation {
     isLoaded = true;
   }
 
-  DillLibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri) {
+  DillLibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri, bool isPatch) {
+    assert(!isPatch);
     return new DillLibraryBuilder(uri, loader);
   }