support constant juxtaposition.

pkg/compiler/lib/src/kernel/constant_visitor.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.md file.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../constants/expressions.dart';
 import 'kernel.dart';
 
 /// Visitor that converts a [ConstantExpression] into a kernel constant
@@ skipping 76 matching lines @@
     throw new UnimplementedError('${exp.toStructuredText()} is not supported.');
   }
 
   @override
   ir.Node visitSymbol(SymbolConstantExpression exp, Kernel kernel) {
     throw new UnimplementedError('${exp.toStructuredText()} is not supported.');
   }
 
   @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
   ir.Node visitConstructed(ConstructedConstantExpression exp, Kernel kernel) {
     List<ir.Expression> positional = <ir.Expression>[];
     List<ir.NamedExpression> named = <ir.NamedExpression>[];
     int positionalCount = exp.callStructure.positionalArgumentCount;
     for (int index = 0; index < positionalCount; index++) {
       ir.Expression argument = visit(exp.arguments[index], kernel);
       if (index < exp.callStructure.positionalArgumentCount) {
@@ skipping 38 matching lines @@
   @override
   ir.Node visitInt(IntConstantExpression exp, Kernel kernel) {
     return new ir.IntLiteral(exp.primitiveValue);
   }
 
   @override
   ir.Node visitBool(BoolConstantExpression exp, Kernel kernel) {
     return new ir.BoolLiteral(exp.primitiveValue);
   }
 }