Use failedAt in more places

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

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index a75c0d2861d492c992d5a22e376ff48ed4e2e47d..7939217b27e0c03920e2bfba3c8b9bf5e72d9eed 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -5331,8 +5331,7 @@ class SsaAstGraphBuilder extends ast.Visitor
   }
 
   visitModifiers(ast.Modifiers node) {
-    throw new SpannableAssertionFailure(
-        node, 'SsaFromAstMixin.visitModifiers not implemented.');
+    failedAt(node, 'SsaFromAstMixin.visitModifiers not implemented.');
   }
 
   visitBreakStatement(ast.BreakStatement node) {
@@ -6456,13 +6455,11 @@ class SsaAstGraphBuilder extends ast.Visitor
   }
 
   visitTypedef(ast.Typedef node) {
-    throw new SpannableAssertionFailure(
-        node, 'SsaFromAstMixin.visitTypedef not implemented.');
+    failedAt(node, 'SsaFromAstMixin.visitTypedef not implemented.');
   }
 
   visitTypeVariable(ast.TypeVariable node) {
-    throw new SpannableAssertionFailure(
-        node, 'SsaFromAstMixin.visitTypeVariable not implemented.');
+    failedAt(node, 'SsaFromAstMixin.visitTypeVariable not implemented.');
   }
 
   /**