Add support for multi-roots

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 1764e5b2bb3e2c31bac01a94fdfcff617e325faa..d259f005c518c90986cf5f3a0848af950de4ff26 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -80,7 +80,11 @@ class SourceLoader<L> extends Loader<L> {
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
-    if (uri == null || uri.scheme != "file") {
+    String scheme = uri?.scheme;
+
+    // TODO(sigmund): source-loader shouldn't check schemes, but defer to the
+    // underlying file system to decide whether it is supported.
+    if (uri == null || (scheme != "file" && scheme != "multi-root")) {
       return inputError(library.uri, -1, "Not found: ${library.uri}.");
     }