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

lib/compiler/implementation/ssa/nodes.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 HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 33 matching lines @@
   R visitMultiply(HMultiply node);
   R visitNegate(HNegate node);
   R visitNot(HNot node);
   R visitParameterValue(HParameterValue node);
   R visitPhi(HPhi node);
   R visitReturn(HReturn node);
   R visitShiftLeft(HShiftLeft node);
   R visitShiftRight(HShiftRight node);
   R visitStatic(HStatic node);
   R visitStaticStore(HStaticStore node);
+  R visitStringConcat(HStringConcat node);
   R visitSubtract(HSubtract node);
   R visitSwitch(HSwitch node);
   R visitThis(HThis node);
   R visitThrow(HThrow node);
   R visitTruncatingDivide(HTruncatingDivide node);
   R visitTry(HTry node);
   R visitTypeGuard(HTypeGuard node);
   R visitTypeConversion(HTypeConversion node);
 }
 
@@ skipping 233 matching lines @@
   visitPhi(HPhi node) => visitInstruction(node);
   visitMultiply(HMultiply node) => visitBinaryArithmetic(node);
   visitParameterValue(HParameterValue node) => visitInstruction(node);
   visitReturn(HReturn node) => visitControlFlow(node);
   visitShiftRight(HShiftRight node) => visitBinaryBitOp(node);
   visitShiftLeft(HShiftLeft node) => visitBinaryBitOp(node);
   visitSubtract(HSubtract node) => visitBinaryArithmetic(node);
   visitSwitch(HSwitch node) => visitControlFlow(node);
   visitStatic(HStatic node) => visitInstruction(node);
   visitStaticStore(HStaticStore node) => visitInstruction(node);
+  visitStringConcat(HStringConcat node) => visitInstruction(node);
   visitThis(HThis node) => visitParameterValue(node);
   visitThrow(HThrow node) => visitControlFlow(node);
   visitTry(HTry node) => visitControlFlow(node);
   visitTruncatingDivide(HTruncatingDivide node) => visitBinaryArithmetic(node);
   visitTypeGuard(HTypeGuard node) => visitCheck(node);
   visitIs(HIs node) => visitInstruction(node);
   visitTypeConversion(HTypeConversion node) => visitCheck(node);
 }
 
 class SubGraph {
@@ skipping 1908 matching lines @@
       sourceElement = input.sourceElement;
   }
 
   HType get guaranteedType() => type;
 
   accept(HVisitor visitor) => visitor.visitTypeConversion(this);
 
   bool hasSideEffects() => checked;
 }
 
+class HStringConcat extends HInstruction {
+  HStringConcat(HInstruction left, HInstrunction right)
+      : super(<HInstruction>[left, right]);
+  HType get guaranteedType() => HType.STRING;
+
+  HInstruction get left() => inputs[0];
+  HInstruction get right() => inputs[1];
+
+  accept(HVisitor visitor) => visitor.visitStringConcat(this);
+  toString() => "string concat";
+}
+
 /** Non-block-based (aka. traditional) loop information. */
 class HLoopInformation {
   final HBasicBlock header;
   final List<HBasicBlock> blocks;
   final List<HBasicBlock> backEdges;
   final List<LabelElement> labels;
   final TargetElement target;
 
   /** Corresponding block information for the loop. */
   HLoopBlockInformation loopBlockInformation;
@@ skipping 304 matching lines @@
   HBasicBlock get start() => expression.start;
   HBasicBlock get end() {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }