Enable conditional directives by default.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 75f1c0899b862ae1a3fa93c75ceee31a9df9f8b9..336dfc6f59ad3f416e7b1e6cb8ef5950ca544b4a 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -318,7 +318,6 @@ Future<api.CompilationResult> compile(List<String> argv) {
     new OptionHandler('--out=.+|-o.*', setOutput, multipleArguments: true),
     new OptionHandler(Flags.allowMockCompilation, passThrough),
     new OptionHandler(Flags.fastStartup, passThrough),
-    new OptionHandler(Flags.conditionalDirectives, passThrough),
     new OptionHandler(Flags.genericMethodSyntax, passThrough),
     new OptionHandler('${Flags.minify}|-m', implyCompilation),
     new OptionHandler(Flags.preserveUris, passThrough),
@@ -356,6 +355,10 @@ Future<api.CompilationResult> compile(List<String> argv) {
     new OptionHandler(Flags.useContentSecurityPolicy, passThrough),
     new OptionHandler(Flags.enableExperimentalMirrors, passThrough),
     new OptionHandler(Flags.enableAssertMessage, passThrough),
+    // TODO(floitsch): remove conditional directives flag.
+    // We don't provide the info-message yet, since we haven't publicly
+    // launched the feature yet.
+    new OptionHandler(Flags.conditionalDirectives, (_) {}),
     new OptionHandler('--enable-async', (_) {
       diagnosticHandler.info(
           "Option '--enable-async' is no longer needed. "