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

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 7788309d1c0e1e6e13308944709b0e9f35187375..b02947b01d2e6acd3899fb217a820aefbff014d6 100644
--- a/pkg/front_end/lib/src/fasta/compiler_command_line.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_command_line.dart
@@ -16,6 +16,7 @@ const Map<String, dynamic> optionSpecification = const <String, dynamic>{
   "--output": Uri,
   "--packages": Uri,
   "--platform": Uri,
+  "--target": String,
   "-o": Uri,
 };
 
@@ -55,6 +56,9 @@ class CompilerCommandLine extends CommandLine {
     } 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 {
@@ -73,6 +77,9 @@ class CompilerCommandLine extends CommandLine {
 
   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>[]);
   }
@@ -177,6 +184,12 @@ Supported options:
   --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