Lesser direct Compiler use in SSA

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

Index: pkg/compiler/lib/src/ssa/kernel_string_builder.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_string_builder.dart b/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
index ddef20aa35bf7412e7eeddfacb57dd7e67d7bf77..f051ba05a62e3ba255cba1870d31dc63a1502ab8 100644
--- a/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
@@ -4,8 +4,7 @@
 
 import 'package:kernel/ast.dart' as ir;
 
-import '../compiler.dart';
-
+import '../common.dart';
 import 'builder_kernel.dart';
 import 'kernel_ast_adapter.dart';
 import 'nodes.dart';
@@ -13,7 +12,6 @@ import 'nodes.dart';
 /// Visits and concatenates the expressions in a string concatenation.
 class KernelStringBuilder extends ir.Visitor {
   final KernelSsaBuilder builder;
-  Compiler get compiler => builder.compiler;
   KernelAstAdapter get astAdapter => builder.astAdapter;
 
   /// The string value generated so far.
@@ -23,8 +21,8 @@ class KernelStringBuilder extends ir.Visitor {
 
   @override
   void defaultNode(ir.Node node) {
-    compiler.reporter
-        .internalError(astAdapter.getNode(node), 'Unexpected node.');
+    throw new SpannableAssertionFailure(
+        astAdapter.getNode(node), 'Unexpected node.');
   }
 
   @override