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

lib/compiler/implementation/ssa/nodes.dart

Index: lib/compiler/implementation/ssa/nodes.dart
diff --git a/lib/compiler/implementation/ssa/nodes.dart b/lib/compiler/implementation/ssa/nodes.dart
index 8920ea03b826194960c438a5441f72dfa6d4b0e0..e67084dd79de649bd9eecbbf8eff615a501691f7 100644
--- a/lib/compiler/implementation/ssa/nodes.dart
+++ b/lib/compiler/implementation/ssa/nodes.dart
@@ -51,6 +51,7 @@ interface HVisitor<R> {
   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);
@@ -304,6 +305,7 @@ class HBaseVisitor extends HGraphVisitor implements HVisitor {
   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);
@@ -2232,6 +2234,18 @@ class HTypeConversion extends HCheck {
   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;