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

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

Index: pkg/front_end/lib/src/fasta/fasta.dart
diff --git a/pkg/front_end/lib/src/fasta/fasta.dart b/pkg/front_end/lib/src/fasta/fasta.dart
index a09e53d53f10734e7fa875548ef42900081c97c3..ecd2c512f7dc9398accca96217b06050abdecff0 100644
--- a/pkg/front_end/lib/src/fasta/fasta.dart
+++ b/pkg/front_end/lib/src/fasta/fasta.dart
@@ -110,6 +110,10 @@ class CompileTask {
 
   CompileTask(this.c, this.ticker);
 
+  DillTarget createDillTarget(TranslateUri uriTranslator) {
+    return new DillTarget(ticker, uriTranslator);
+  }
+
   KernelTarget createKernelTarget(
       DillTarget dillTarget, TranslateUri uriTranslator, bool strongMode) {
     return new KernelTarget(
@@ -120,7 +124,7 @@ class CompileTask {
     TranslateUri uriTranslator =
         await TranslateUri.parse(c.options.sdk, c.options.packages);
     ticker.logMs("Read packages file");
-    DillTarget dillTarget = new DillTarget(ticker, uriTranslator);
+    DillTarget dillTarget = createDillTarget(uriTranslator);
     KernelTarget kernelTarget =
         createKernelTarget(dillTarget, uriTranslator, c.options.strongMode);
     if (c.options.strongMode) {