Simplify generated code for string interpolation by delaying the constant folding.

lib/compiler/implementation/ssa/tracer.dart

 // Copyright (c) 2011, 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('tracer');
 
 #import('dart:io');
 #import('ssa.dart');
 #import('../leg.dart');
 
@@ skipping 363 matching lines @@
 
   String visitShiftRight(HShiftRight node) => visitInvokeStatic(node);
 
   String visitStatic(HStatic node)
       => "Static ${node.element.name.slowToString()}";
   String visitStaticStore(HStaticStore node) {
     String lhs = node.element.name.slowToString();
     return "Static $lhs = ${temporaryId(node.inputs[0])}";
   }
 
+  String visitStringConcat(HStringConcat node) {
+    var leftId = temporaryId(node.left);
+    var rightId = temporaryId(node.right);
+    return "StringConcat: $leftId + $rightId";
+  }
+
   String visitSubtract(HSubtract node) => visitInvokeStatic(node);
 
   String visitSwitch(HSwitch node) {
     StringBuffer buf = new StringBuffer();
     buf.add("Switch: (");
     buf.add(temporaryId(node.inputs[0]));
     buf.add(") ");
     for (int i = 1; i < node.inputs.length; i++) {
       buf.add(temporaryId(node.inputs[i]));
       buf.add(": B");
@@ skipping 63 matching lines @@
   String visitIs(HIs node) {
     String type = node.typeExpression.toString();
     return "TypeTest: ${temporaryId(node.expression)} is $type";
   }
 
   String visitTypeConversion(HTypeConversion node) {
     String type = node.propagatedType.toString();
     return "TypeConversion: ${temporaryId(node.inputs[0])} to $type";
   }
 }