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

lib/compiler/implementation/ssa/optimize.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.
 
 interface OptimizationPhase {
   String get name();
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 572 matching lines @@
 
   HInstruction visitInvokeDynamicSetter(HInvokeDynamicSetter node) {
     HInstruction receiver = node.inputs[0];
     if (!receiver.propagatedType.isUseful()) return node;
     Type type = receiver.propagatedType.computeType(compiler);
     if (type === null) return node;
     Element field = compiler.world.locateSingleField(type, node.name);
     if (field === null) return node;
     return new HFieldSet(field, node.inputs[0], node.inputs[1]);
   }
+
+  HInstruction visitStringConcat(HStringConcat node) {
+    DartString folded = const LiteralDartString("");
+    for (int i = 0; i < node.inputs.length; i++) {
+      HInstruction part = node.inputs[i];
+      if (!part.isConstant()) return node;
+      HConstant constant = part;
+      if (!constant.constant.isPrimitive()) return node;
+      PrimitiveConstant primitive = constant.constant;
+      folded = new DartString.concat(folded, primitive.toDartString());
+    }
+    return graph.addConstantString(folded, node);
+  }
 }
 
 class SsaCheckInserter extends HBaseVisitor implements OptimizationPhase {
   final String name = "SsaCheckInserter";
   Element lengthInterceptor;
 
   SsaCheckInserter(JavaScriptBackend backend) {
     SourceString lengthString = const SourceString('length');
     lengthInterceptor =
         backend.builder.interceptors.getStaticGetInterceptor(lengthString);
@@ skipping 521 matching lines @@
         // the if block terminates. So any use of the instruction
         // after the join block should be changed to the new
         // instruction.
         changeUsesDominatedBy(ifUser.joinBlock, input, convertedType);
       }
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }