support constant juxtaposition.

pkg/compiler/lib/src/kernel/constant_visitor.dart

Index: pkg/compiler/lib/src/kernel/constant_visitor.dart
diff --git a/pkg/compiler/lib/src/kernel/constant_visitor.dart b/pkg/compiler/lib/src/kernel/constant_visitor.dart
index 0d1e1a634fde9ee43cdbc25e0a3a958adafb5292..3d0405d8f6a81d6cf8b7419e2b4bc1b1c786f9c7 100644
--- a/pkg/compiler/lib/src/kernel/constant_visitor.dart
+++ b/pkg/compiler/lib/src/kernel/constant_visitor.dart
@@ -94,7 +94,8 @@ class ConstantVisitor extends ConstantExpressionVisitor<ir.Node, Kernel> {
 
   @override
   ir.Node visitConcatenate(ConcatenateConstantExpression exp, Kernel kernel) {
-    throw new UnimplementedError('${exp.toStructuredText()} is not supported.');
+    return new ir.StringConcatenation(
+        exp.expressions.map((e) => visit(e, kernel)).toList());
   }
 
   @override