Fix command line args validation for platform compilation with fasta

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 CommandLine, argumentError;
 
@@ skipping 32 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 (programName == "compile_platform" && arguments.length != 2) {
+    if (programName == "compile_platform" && arguments.length != 3) {
+      // The three expected arguments are: patched_sdk/lib path, full output
+      // path (usually "platform.dill"), and outline output path.
       return argumentError(usage, "Expected two arguments.");

[ahe, 2017/05/15 11:02:01]

Consider removing the comment and updating the error message.

[Dmitry Stefantsov, 2017/05/15 11:07:03]

Right :) Thanks!

     } 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");
@@ skipping 49 matching lines @@
       summary =
           "Compiles a Dart program to the Dill/Kernel IR format via dartk.";
       break;
 
     case "run":
       summary = "Runs a Dart program.";
       break;
 
     case "compile_platform":
       summary = "Compiles Dart SDK platform to the Dill/Kernel IR format.";
-      basicUsage = "Usage: $programName [options] patched_sdk output\n";
+      basicUsage = "Usage: $programName [options] patched_sdk fullOutput "
+          "outlineOutput\n";
   }
   StringBuffer sb = new StringBuffer(basicUsage);
   if (summary != null) {
     sb.writeln();
     sb.writeln(summary);
     sb.writeln();
   }
   sb.write(options);
   return "$sb";
 }
@@ skipping 43 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.
 """;