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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta;
 
 import 'dart:async' show Future;
 
 import 'dart:convert' show JSON;
 
@@ skipping 115 matching lines @@
         c.fileSystem, c.options.sdk, c.options.packages);
     ticker.logMs("Read packages file");
     DillTarget dillTarget = createDillTarget(uriTranslator);
     KernelTarget kernelTarget =
         createKernelTarget(dillTarget, uriTranslator, c.options.strongMode);
     if (c.options.strongMode) {
       print("Note: strong mode support is preliminary and may not work.");
     }
     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);
     String path = uriTranslator.translate(uri)?.path ?? argument;
     if (path.endsWith(".dart")) {
       kernelTarget.read(uri);
     } else {
       inputError(uri, -1, "Unexpected input: $uri");
     }
     await dillTarget.writeOutline(null);
@@ skipping 130 matching lines @@
   final BytesBuilder builder = new BytesBuilder();
 
   void add(List<int> data) {
     builder.add(data);
   }
 
   void close() {
     // Nothing to do.
   }
 }