Rework DillLoader to allow adding multiple dills.

pkg/kernel/test/interpreter/suite.dart

Index: pkg/kernel/test/interpreter/suite.dart
diff --git a/pkg/kernel/test/interpreter/suite.dart b/pkg/kernel/test/interpreter/suite.dart
index 2d6fb08391d2f4fbf4ccb1e070afe5973abdabbc..4ebbe084d9cfcc36445a5e8a8e69fe045eac975b 100644
--- a/pkg/kernel/test/interpreter/suite.dart
+++ b/pkg/kernel/test/interpreter/suite.dart
@@ -76,9 +76,7 @@ class FastaCompile extends Step<TestDescription, Program, InterpreterContext> {
     Program platform = await context.loadPlatform();
     Ticker ticker = new Ticker();
     DillTarget dillTarget = new DillTarget(ticker, context.uriTranslator);
-    dillTarget.loader
-      ..input = Uri.parse("org.dartlang:platform") // Make up a name.
-      ..setProgram(platform);
+    dillTarget.loader.appendLibraries(platform);
     KernelTarget sourceTarget = new KernelTarget(PhysicalFileSystem.instance,
         dillTarget, context.uriTranslator, context.strongMode);