Add support for building patched_sdk and platform.dill for dart2js:

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,
+  "--target": String,
   "-o": Uri,
 };
 
 class CompilerCommandLine extends CommandLine {
   final String programName;
 
   CompilerCommandLine(String programName, List<String> arguments)
       : programName = programName,
         super(arguments,
             specification: optionSpecification,
@@ skipping 19 matching lines @@
     }
 
     if (options.containsKey("-o") && options.containsKey("--output")) {
       return argumentError(usage, "Can't specify both '-o' and '--output'.");
     }
     if (programName == "compile_platform" && arguments.length != 2) {
       return argumentError(usage, "Expected two arguments.");
     } else if (arguments.isEmpty) {
       return argumentError(usage, "No Dart file specified.");
     }
+    if (!targetDart2js && !targetVm) {
+      return argumentError(usage, "Invalid target");

[ahe, 2017/04/27 13:34:34]

"Unknown target: '${options["--target"]}'."

[Siggi Cherem (dart-lang), 2017/04/28 21:37:20]

n/a anymore (flag is gone)

+    }
   }
 
   Uri get output {
     return options["-o"] ?? options["--output"] ?? defaultOutput;
   }
 
   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 packages => options["--packages"] ?? Uri.base.resolve(".packages");
 
   Uri get sdk => options["--compile-sdk"];
 
+  bool get targetDart2js => options["--target"] == 'dart2js';
+  bool get targetVm => (options["--target"] ?? 'vm') == 'vm';
+
   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 84 matching lines @@
 
   --dump-ir
     Print compiled libraries in Kernel source notation.
 
   --exclude-source
     Do not include source code in the dill file.
 
   --compile-sdk=<patched_sdk>
     Compile the SDK from scratch instead of reading it from 'platform.dill'.
 
+  --target=vm
+  --target=dart2js
+    Compile with the intention to pass the output to the given target. This will
+    affect which patch files are used for compiling the SDK and what
+    transformations will be done in the Kernel representation.
+
   --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.
 """;