Rework DillLoader to allow adding multiple dills.

pkg/kernel/test/closures/suite.dart

Index: pkg/kernel/test/closures/suite.dart
diff --git a/pkg/kernel/test/closures/suite.dart b/pkg/kernel/test/closures/suite.dart
index dd4419d4d8433f2f06bb5ce4416c1d5d7d47a1b2..f79cb2305e0facc8ffa7a04212ccba2d8a531c9a 100644
--- a/pkg/kernel/test/closures/suite.dart
+++ b/pkg/kernel/test/closures/suite.dart
@@ -60,7 +60,8 @@ class ClosureConversionContext extends ChainContext {
 
   Future<Program> loadPlatform() async {
     Uri sdk = await computePatchedSdk();
-    return loadProgramFromBinary(sdk.resolve('platform.dill').toFilePath());
+    String platformPath = sdk.resolve('platform.dill').toFilePath();
+    return loadProgramFromBinary(platformPath);
   }
 
   static Future<ClosureConversionContext> create(
@@ -93,9 +94,7 @@ class FastaCompile
     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.setProgram(platform);
     KernelTarget sourceTarget = new KernelTarget(PhysicalFileSystem.instance,
         dillTarget, context.uriTranslator, context.strongMode);