Add option --compile-sdk to fasta compile and run.

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 43 matching lines @@
     }
   }
 
   Uri get output {
     return options["-o"] ?? options["--output"] ?? defaultOutput;
   }
 
   Uri get defaultOutput => Uri.base.resolve("${arguments.first}.dill");
 
   Uri get platform {
-    return options["--platform"] ?? Uri.base.resolve("platform.dill");
+    return options.containsKey("--compile-sdk")
+        ? null
+        : options["--platform"] ?? Uri.base.resolve("platform.dill");
   }
 }
 
 String computeUsage(String programName, bool verbose) {
   String basicUsage = "Usage: $programName [options] dartfile\n";
   String summary;
   String options = (verbose ? allOptions : frequentOptions).trim();
   switch (programName) {
     case "outline":
       summary =
@@ skipping 53 matching lines @@
   --platform=<file>
     Read the SDK platform from <file>, which should be in Dill/Kernel IR format
     and contain the Dart SDK.
 
   --verify
     Check that the generated output is free of various problems. This is mostly
     useful for developers of this compiler or Kernel transformations.
 
   --dump-ir
     Print compiled libraries in Kernel source notation.
+
+  --compile-sdk
+    Compile the SDK from scratch instead of reading it from 'platform.dill'.
 """;