make kerneltask an actual task

pkg/compiler/lib/src/js_backend/kernel_task.dart

-// Copyright (c) 2016, 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.
-
-import '../compiler.dart';
-import '../common/names.dart';
-import '../elements/elements.dart';
-import '../kernel/kernel.dart';
-import 'package:kernel/ast.dart' as ir;
-
-import 'backend.dart';
-
-/// Visits the compiler main function and builds the kernel representation.
-///
-/// This creates a mapping from kernel nodes to AST nodes to be used later.
-class KernelTask {
-  final Compiler _compiler;
-  final Kernel kernel;
-
-  KernelTask(JavaScriptBackend backend)
-      : this._compiler = backend.compiler,
-        this.kernel = new Kernel(backend.compiler);
-
-  ir.Program program;
-
-  /// Builds the kernel IR for the main function.
-  ///
-  /// May enqueue more elements to the resolution queue.
-  void buildKernelIr() {
-    program = buildProgram(_compiler.mainApp);
-  }
-
-  /// Builds the kernel IR program for the main function exported from
-  /// [library].
-  ///
-  /// May enqueue more elements to the resolution queue.
-  ir.Program buildProgram(LibraryElement library) {
-    return new ir.Program(kernel.libraryDependencies(library.canonicalUri))
-      ..mainMethod =
-          kernel.functionToIr(library.findExported(Identifiers.main));
-  }
-}