Add KernelToElementMap.getSpannable to avoid most usage of KernelAstAdapter.getNode

pkg/compiler/lib/src/ssa/ssa_branch_builder.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 '../common.dart';
 import '../io/source_information.dart';
 import '../tree/tree.dart' as ast;
 
 import 'graph_builder.dart';
 import 'locals_handler.dart';
 import 'nodes.dart';
 
 class SsaBranch {
   final SsaBranchBuilder branchBuilder;
   final HBasicBlock block;
   LocalsHandler startLocals;
   LocalsHandler exitLocals;
   SubGraph graph;
 
   SsaBranch(this.branchBuilder) : block = new HBasicBlock();
 }
 
 class SsaBranchBuilder {
   final GraphBuilder builder;
-  final ast.Node diagnosticNode;
+  final Spannable diagnosticNode;
 
   SsaBranchBuilder(this.builder, [this.diagnosticNode]);
 
   void checkNotAborted() {
     if (builder.isAborted()) {
       throw new SpannableAssertionFailure(
           diagnosticNode, "aborted control flow");
     }
   }
 
@@ skipping 183 matching lines @@
         new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }