Support general handling of type variables by substituting types into the current context.

sdk/lib/_internal/compiler/implementation/dart2jslib.dart

Index: sdk/lib/_internal/compiler/implementation/dart2jslib.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2jslib.dart b/sdk/lib/_internal/compiler/implementation/dart2jslib.dart
index 382110701684742e5fd066c24886a398f097f569..58e9279f299d80da30fd679ea63c4fa3782e2aea 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2jslib.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2jslib.dart
@@ -21,6 +21,7 @@ import 'elements/modelx.dart'
          AnalyzableElement,
          DeferredLoaderGetterElementX,
          SynthesizedCallMethodElementX;
+import 'helpers/helpers.dart';
 import 'js_backend/js_backend.dart' as js_backend;
 import 'native_handler.dart' as native;
 import 'scanner/scannerlib.dart';