Make patch_sdk step invoke fasta's compile_platform to generate patched_sdk/platform.dill.

pkg/front_end/lib/src/fasta/compiler_command_line.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.compiler_command_line;
 
 import 'dart:io' show
     exit;
 
 import 'command_line.dart' show
@@ skipping 33 matching lines @@
 
   void validate() {
     if (help) {
       print(computeUsage(programName, verbose));
       exit(0);
     }
 
     if (options.containsKey("-o") && options.containsKey("--output")) {
       return argumentError(usage, "Can't specify both '-o' and '--output'.");
     }
-    if (options.containsKey("--packages")) {
-      return argumentError(usage, "Option '--packages' isn't supported yet.");
-    }
     if (programName == "compile_platform" && arguments.length != 2) {
       return argumentError(usage, "Expected two arguments.");
     } else if (arguments.isEmpty) {
       return argumentError(usage, "No Dart file specified.");
     }
   }
 
   Uri get output {
     return options["-o"] ?? options["--output"] ?? defaultOutput;
   }
 
   Uri get defaultOutput => Uri.base.resolve("${arguments.first}.dill");
 
   Uri get platform {
     return options.containsKey("--compile-sdk")
         ? null
         : options["--platform"] ?? Uri.base.resolve("platform.dill");
   }
 
+  Uri get packages => options["--packages"] ?? Uri.base.resolve(".packages");
+
   Uri get sdk => options["--compile-sdk"];
 
   Set<String> get fatal {
     return new Set<String>.from(options["--fatal"] ?? <String>[]);
   }
 
   bool get errorsAreFatal => fatal.contains("errors");
 
   bool get warningsAreFatal => fatal.contains("warnings");
 
@@ skipping 90 matching lines @@
     Compile the SDK from scratch instead of reading it from 'platform.dill'.
 
   --fatal=errors
   --fatal=warnings
   --fatal=nits
     Makes messages of the given kinds fatal, that is, immediately stop the
     compiler with a non-zero exit-code. In --verbose mode, also display an
     internal stack trace from the compiler. Multiple kinds can be separated by
     commas, for example, --fatal=errors,warnings.
 """;