Fix command line args validation for platform compilation with fasta

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

Index: pkg/front_end/lib/src/fasta/compiler_command_line.dart
diff --git a/pkg/front_end/lib/src/fasta/compiler_command_line.dart b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
index 7788309d1c0e1e6e13308944709b0e9f35187375..ab6d6a47ebbeddd186d63d47e8bfb82dcb92b8cb 100644
--- a/pkg/front_end/lib/src/fasta/compiler_command_line.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
@@ -50,7 +50,9 @@ class CompilerCommandLine extends CommandLine {
     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.");
@@ -121,7 +123,8 @@ String computeUsage(String programName, bool verbose) {
 
     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) {