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

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 8875d282e4e758faf4a9939bfde4a18e9482da86..2413488ea7a3f6f6db6f4cff02130b882f52c465 100644
--- a/pkg/front_end/lib/src/fasta/compiler_command_line.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
@@ -61,7 +61,9 @@ class CompilerCommandLine extends CommandLine {
   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");
   }
 }
 
@@ -135,4 +137,7 @@ Supported options:
 
   --dump-ir
     Print compiled libraries in Kernel source notation.
+
+  --compile-sdk
+    Compile the SDK from scratch instead of reading it from 'platform.dill'.
 """;