Specify patched_sdk location with --compile-sdk.

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;
 
 import 'compiler_context.dart' show
     CompilerContext;
 
 const Map<String, dynamic> optionSpecification = const <String, dynamic>{
+  "--compile-sdk": Uri,
+  "--fatal": ",",
+  "--output": Uri,
+  "--packages": Uri,
+  "--platform": Uri,
   "-o": Uri,
-  "--output": Uri,
-  "--platform": Uri,
-  "--packages": Uri,
-  "--fatal": ",",
 };
 
 class CompilerCommandLine extends CommandLine {
   final String programName;
 
   CompilerCommandLine(String programName, List<String> arguments)
       : programName = programName,
         super(arguments, specification: optionSpecification,
             usage: computeUsage(programName, false));
 
@@ skipping 30 matching lines @@
   }
 
   Uri get defaultOutput => Uri.base.resolve("${arguments.first}.dill");
 
   Uri get platform {
     return options.containsKey("--compile-sdk")
         ? null
         : options["--platform"] ?? Uri.base.resolve("platform.dill");
   }
 
+  Uri get sdk => options["--compile-sdk"];
+
   Set<String> get fatal {
     return new Set<String>.from(options["--fatal"] ?? <String>[]);
   }
 
   bool get errorsAreFatal => fatal.contains("errors");
 
   bool get warningsAreFatal => fatal.contains("warnings");
 
   bool get nitsAreFatal => fatal.contains("nits");
 
@@ skipping 72 matching lines @@
     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-sdk=<patched_sdk>
     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.
 """;