Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 9d14dc18233e5a4876e45b14e64d79814a68f4ba..ba2bbee6fa583551d08411d579a7869d22a05017 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -814,7 +814,7 @@ class DeferredLoadTask extends CompilerTask {
   void beforeResolution(LibraryEntity mainLibrary) {
     if (mainLibrary == null) return;
     // TODO(johnniwinther): Support deferred load for kernel based elements.
-    if (compiler.options.loadFromDill) return;
+    if (compiler.options.previewDart2) return;
     _allDeferredImports[_fakeMainImport] = mainLibrary;
     var lastDeferred;
     // When detecting duplicate prefixes of deferred libraries there are 4