kernel->ssa: support string concatenations/interpolations

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

Index: pkg/compiler/lib/src/ssa/builder_kernel.dart
diff --git a/pkg/compiler/lib/src/ssa/builder_kernel.dart b/pkg/compiler/lib/src/ssa/builder_kernel.dart
index 031bdab1f5b7a2f39d4c62b4fd3b8b1fb8fb392e..2932babef1c765b1003acc19022d42d8da65460d 100644
--- a/pkg/compiler/lib/src/ssa/builder_kernel.dart
+++ b/pkg/compiler/lib/src/ssa/builder_kernel.dart
@@ -19,6 +19,7 @@ import '../universe/selector.dart';
 
 import 'graph_builder.dart';
 import 'kernel_ast_adapter.dart';
+import 'kernel_string_builder.dart';
 import 'locals_handler.dart';
 import 'nodes.dart';
 import 'ssa_branch_builder.dart';
@@ -565,4 +566,11 @@ class KernelSsaBuilder extends ir.Visitor with GraphBuilder {
     not.operand.accept(this);
     push(new HNot(popBoolified(), backend.boolType));
   }
+
+  @override
+  void visitStringConcatenation(ir.StringConcatenation stringConcat) {
+    KernelStringBuilder stringBuilder = new KernelStringBuilder(this);
+    stringConcat.accept(stringBuilder);
+    stack.add(stringBuilder.result);
+  }
 }