Implement deps files in compile_platform.dart.

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 cbd90b6b432f54c9334ff37b6a363532f78d6101..5e508462945b3ac4b52d8aadfc046d5f5989957a 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -341,4 +341,6 @@ class SourceLoader<L> extends Loader<L> {
     coreTypes = new CoreTypes(program);
     ticker.logMs("Computed core types");
   }
+
+  List<Uri> getDependencies() => sourceBytes.keys.toList();
 }