Rework DillLoader to allow adding multiple dills.

pkg/kernel/lib/kernel.dart

Index: pkg/kernel/lib/kernel.dart
diff --git a/pkg/kernel/lib/kernel.dart b/pkg/kernel/lib/kernel.dart
index 752617b9d3fc2ed5ee541fd11667fa9026c92417..48af8f45c6d20bc1c52f226c4a46771aa559fc1a 100644
--- a/pkg/kernel/lib/kernel.dart
+++ b/pkg/kernel/lib/kernel.dart
@@ -23,8 +23,13 @@ import 'text/ast_to_text.dart';
 export 'ast.dart';
 
 Program loadProgramFromBinary(String path, [Program program]) {
+  List<int> bytes = new File(path).readAsBytesSync();
+  return loadProgramFromBytes(bytes, program);
+}
+
+Program loadProgramFromBytes(List<int> bytes, [Program program]) {
   program ??= new Program();
-  new BinaryBuilder(new File(path).readAsBytesSync()).readProgram(program);
+  new BinaryBuilder(bytes).readProgram(program);
   return program;
 }