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

pkg/front_end/lib/src/fasta/source/diet_listener.dart

Index: pkg/front_end/lib/src/fasta/source/diet_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/diet_listener.dart b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
index 36b7c6ce224ee725fb4dad5facfd5a1bd001090b..695b5fe2e36dfd2a79775e8e16b7196c8a52d55d 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -22,8 +22,7 @@ import '../parser/parser.dart' show Parser, optional;
 
 import '../scanner/token.dart' show BeginGroupToken, Token;
 
-import '../parser/dart_vm_native.dart'
-    show removeNativeClause, skipNativeClause;
+import '../parser/dart_vm_native.dart' show removeNativeClause;
 
 import '../util/link.dart' show Link;
 
@@ -483,11 +482,8 @@ class DietListener extends StackListener {
   @override
   Token handleUnrecoverableError(Token token, FastaMessage message) {
     if (isDartLibrary && message.code == codeExpectedBlockToSkip) {
-      Token recover = skipNativeClause(token);
-      if (recover != null) {
-        assert(isTargetingDartVm);
-        return recover;
-      }
+      Token recover = library.loader.target.skipNativeClause(token);
+      if (recover != null) return recover;
     }
     return super.handleUnrecoverableError(token, message);
   }
@@ -569,11 +565,6 @@ class DietListener extends StackListener {
     return builder;
   }
 
-  bool get isTargetingDartVm {
-    // TODO(ahe): Find a more reliable way to check if this is the Dart VM.
-    return !coreTypes.containsLibrary("dart:_js_helper");
-  }
-
   @override
   void debugEvent(String name) {
     // printEvent(name);