Use failedAt in more places

pkg/compiler/lib/src/ssa/kernel_string_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 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import 'builder_kernel.dart';
 import 'nodes.dart';
 
 /// Visits and concatenates the expressions in a string concatenation.
 class KernelStringBuilder extends ir.Visitor {
   final KernelSsaGraphBuilder builder;
 
   /// The string value generated so far.
   HInstruction result = null;
 
   KernelStringBuilder(this.builder);
 
   @override
   void defaultNode(ir.Node node) {
-    throw new SpannableAssertionFailure(
-        CURRENT_ELEMENT_SPANNABLE, 'Unexpected node: $node');
+    failedAt(CURRENT_ELEMENT_SPANNABLE, 'Unexpected node: $node');
   }
 
   @override
   void defaultExpression(ir.Expression node) {
     node.accept(builder);
     HInstruction expression = builder.pop();
 
     // We want to use HStringify when:
     //   1. The value is known to be a primitive type, because it might get
     //      constant-folded and codegen has some tricks with JavaScript
@@ skipping 29 matching lines @@
     return instruction;
   }
 
   HInstruction stringify(HInstruction expression) {
     HInstruction instruction =
         new HStringify(expression, builder.commonMasks.stringType);
     builder.add(instruction);
     return instruction;
   }
 }