cps-ir: Implement string concatenation.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

 // Copyright (c) 2014, 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 file.
 
 library tree_ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, GenericType;
 import '../elements/elements.dart';
@@ skipping 163 matching lines @@
 }
 
 /**
  * Call to a factory or generative constructor.
  */
 class InvokeConstructor extends Expression implements Invoke {
   final DartType type;
   final FunctionElement target;
   final List<Expression> arguments;
   final Selector selector;
+  /// TODO(karlklose): get rid of this field.  Instead use the constant's
+  /// expression to find the constructor to be called in dart2dart.
   final values.ConstantValue constant;
 
   InvokeConstructor(this.type, this.target, this.selector, this.arguments,
       [this.constant]);
 
   ClassElement get targetClass => target.enclosingElement;
 
   accept(ExpressionVisitor visitor) => visitor.visitInvokeConstructor(this);
   accept1(ExpressionVisitor1 visitor, arg) {
     return visitor.visitInvokeConstructor(this, arg);
   }
 }
 
 /// Calls [toString] on each argument and concatenates the results.
 class ConcatenateStrings extends Expression {
   final List<Expression> arguments;
-  final values.ConstantValue constant;
 
-  ConcatenateStrings(this.arguments, [this.constant]);
+  ConcatenateStrings(this.arguments);
 
   accept(ExpressionVisitor visitor) => visitor.visitConcatenateStrings(this);
   accept1(ExpressionVisitor1 visitor, arg) {
     return visitor.visitConcatenateStrings(this, arg);
   }
 }
 
 /**
  * A constant.
  */
@@ skipping 706 matching lines @@
     visitStatement(node.next);
   }
 
   visitCreateBox(CreateBox node) {
   }
 
   visitCreateInstance(CreateInstance node) {
     node.arguments.forEach(visitExpression);
   }
 }