Rework DillLoader to allow adding multiple dills.

pkg/front_end/lib/src/fasta/dill/dill_target.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_target.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_target.dart b/pkg/front_end/lib/src/fasta/dill/dill_target.dart
index 4e53c689b4a8f9b47f3d53314330587d58b78401..fff15fb37fb273ae4a3dd924498bb93addc80d2a 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_target.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_target.dart
@@ -37,11 +37,7 @@ class DillTarget extends TargetImplementation {
   }
 
   void read(Uri uri) {
-    if (loader.input == null) {
-      loader.input = uri;
-    } else {
-      inputError(uri, -1, "Can only read one dill file.");
-    }
+    internalError("not implemented");

[ahe, 2017/05/11 10:09:49]

"Internal error: Unsupported operation."

   }
 
   Future<Null> writeProgram(Uri uri) {
@@ -49,7 +45,7 @@ class DillTarget extends TargetImplementation {
   }
 
   Future<Null> writeOutline(Uri uri) async {
-    if (loader.input == null) return null;
+    if (loader.program == null) return null;
     await loader.buildOutlines();
     isLoaded = true;
     return null;