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

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 490c5002e9ad6e279e16028c8281a69feeaeb2e2..8a328d6d26670d4c30c00d241e8b35e2a7005a56 100644
--- a/pkg/front_end/lib/src/fasta/compiler_command_line.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
@@ -51,9 +51,6 @@ class CompilerCommandLine extends CommandLine {
     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) {
@@ -73,6 +70,8 @@ class CompilerCommandLine extends CommandLine {
         : 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 {