Read SDK and patches from a JSON file.

pkg/front_end/lib/src/fasta/loader.dart

Index: pkg/front_end/lib/src/fasta/loader.dart
diff --git a/pkg/front_end/lib/src/fasta/loader.dart b/pkg/front_end/lib/src/fasta/loader.dart
index ba758acab6dbf567cc414e4644bed689834c52d3..306d163f4b9c1b6937c4fba606b9f2dcfbf19263 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -49,7 +49,7 @@ abstract class Loader<L> {
   /// directive. If [accessor] isn't allowed to access [uri], it's a
   /// compile-time error.
   LibraryBuilder read(Uri uri, int charOffset,
-      {Uri fileUri, LibraryBuilder accessor}) {
+      {Uri fileUri, LibraryBuilder accessor, bool isPatch: false}) {
     firstSourceUri ??= uri;
     LibraryBuilder builder = builders.putIfAbsent(uri, () {
       if (fileUri == null) {
@@ -64,11 +64,15 @@ abstract class Loader<L> {
             break;
         }
       }
-      LibraryBuilder library = target.createLibraryBuilder(uri, fileUri);
+      LibraryBuilder library =
+          target.createLibraryBuilder(uri, fileUri, isPatch);
       if (uri.scheme == "dart" && uri.path == "core") {
         coreLibrary = library;
         target.loadExtraRequiredLibraries(this);
       }
+      if (uri.scheme == "dart") {
+        target.readPatchFiles(library);
+      }
       first ??= library;
       if (library.loader == this) {
         unparsedLibraries.addLast(library);