Share more logic between the Ssa(Ast)Builder and SsaKernelBuilder

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library ssa;
 
+import '../common/codegen.dart' show CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../elements/elements.dart' show Element, FunctionElement;
 import '../io/source_information.dart';
 import '../js/js.dart' as js;
 import '../js_backend/backend.dart' show JavaScriptBackend, FunctionCompiler;
 import '../js_backend/element_strategy.dart' show ElementCodegenWorkItem;
 import '../world.dart' show ClosedWorld;
 
 import 'builder.dart';
 import 'builder_kernel.dart';
 import 'codegen.dart';
 import 'nodes.dart';
 import 'optimize.dart';
 
 class SsaFunctionCompiler implements FunctionCompiler {
   final SsaCodeGeneratorTask generator;
-  final SsaBuilderTask builder;
-  final SsaKernelBuilderTask builderKernel;
+  final SsaBuilderTask _builder;
   final SsaOptimizerTask optimizer;
   final JavaScriptBackend backend;
-  final bool useKernel;
 
   SsaFunctionCompiler(JavaScriptBackend backend,
-      SourceInformationStrategy sourceInformationFactory, this.useKernel)
+      SourceInformationStrategy sourceInformationFactory, bool useKernel)
       : generator = new SsaCodeGeneratorTask(backend, sourceInformationFactory),
-        builder = new SsaBuilderTask(backend, sourceInformationFactory),
-        builderKernel =
-            new SsaKernelBuilderTask(backend, sourceInformationFactory),
+        _builder = useKernel
+            ? new SsaKernelBuilderTask(backend, sourceInformationFactory)
+            : new SsaAstBuilderTask(backend, sourceInformationFactory),
         optimizer = new SsaOptimizerTask(backend),
         backend = backend;
 
   /// Generates JavaScript code for `work.element`.
   /// Using the ssa builder, optimizer and codegenerator.
   js.Fun compile(ElementCodegenWorkItem work, ClosedWorld closedWorld) {
-    HGraph graph = useKernel
-        ? builderKernel.build(work, closedWorld)
-        : builder.build(work, closedWorld);
+    HGraph graph = _builder.build(work, closedWorld);
+    if (graph == null) return null;
     optimizer.optimize(work, graph, closedWorld);
     Element element = work.element;
     js.Expression result = generator.generateCode(work, graph, closedWorld);
     if (element is FunctionElement) {
       // TODO(sigmund): replace by kernel transformer when `useKernel` is true.
       result = backend.rewriteAsync(element, result);
     }
     return result;
   }
 
   Iterable<CompilerTask> get tasks {
-    return <CompilerTask>[
-      useKernel ? builderKernel : builder,
-      optimizer,
-      generator
-    ];
+    return <CompilerTask>[_builder, optimizer, generator];
   }
 }
+
+abstract class SsaBuilderTask implements CompilerTask {
+  /// Creates the [HGraph] for [work] or returns `null` if no code is needed
+  /// for [work].
+  HGraph build(CodegenWorkItem work, ClosedWorld closedWorld);
+}