Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index f279ee8ada6eec0b6991a2618cf8c51f60e249b9..53db79922c495e3f8c378e5efd0eca059d381d3a 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -377,7 +377,8 @@ class KernelAstAdapter extends KernelElementAdapterMixin {
 
   TypeMask get streamIteratorConstructorType =>
       TypeMaskFactory.inferredReturnTypeForElement(
-          _backend.helpers.streamIteratorConstructor, _globalInferenceResults);
+          _backend.helpers.streamIteratorConstructor as FunctionEntity,
+          _globalInferenceResults);
 
   ir.Procedure get fallThroughError =>
       kernel.functions[_backend.helpers.fallThroughError];
@@ -483,7 +484,7 @@ class KernelAstAdapter extends KernelElementAdapterMixin {
   }
 
   int _extractEnumIndexFromConstantValue(
-      ConstantValue constant, Element classElement) {
+      ConstantValue constant, ClassEntity classElement) {
     if (constant is ConstructedConstantValue) {
       if (constant.type.element == classElement) {
         assert(constant.fields.length == 1);