Remove one more use of DillTarget.read that broke compiler/tool/generate_kernel.

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

Index: pkg/front_end/lib/src/fasta/fasta.dart
diff --git a/pkg/front_end/lib/src/fasta/fasta.dart b/pkg/front_end/lib/src/fasta/fasta.dart
index 6076090b22593cbd3e4b665cf9c4b871c2f60a32..953283c92a6ab51bf7f2aac4f96826c5ade8ac40 100644
--- a/pkg/front_end/lib/src/fasta/fasta.dart
+++ b/pkg/front_end/lib/src/fasta/fasta.dart
@@ -133,7 +133,7 @@ class CompileTask {
     }
     Uri platform = c.options.platform;
     if (platform != null) {
-      dillTarget.read(platform);
+      _appendDillForUri(dillTarget, platform);
     }
     String argument = c.options.arguments.first;
     Uri uri = Uri.base.resolve(argument);