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 6f4bbb730890e45ff9d2a58aa1601802e9d056e6..aca04d3131e58d72a063f3f7da7d48eece04aee8 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -43,7 +43,7 @@ abstract class Loader<L> {
   ///
   /// Canonical URIs have schemes like "dart", or "package", and the actual
   /// location is often a file URI.
-  LibraryBuilder read(Uri uri, [Uri fileUri]) {
+  LibraryBuilder read(Uri uri, {Uri fileUri, bool isPatch: false}) {
     firstSourceUri ??= uri;
     LibraryBuilder builder = builders.putIfAbsent(uri, () {
       if (fileUri == null) {
@@ -58,11 +58,15 @@ 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);