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

lib/compiler/implementation/ssa/codegen.dart

 // Copyright (c) 2012, 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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   final JavaScriptBackend backend;
   SsaCodeGeneratorTask(JavaScriptBackend backend)
       : this.backend = backend,
         super(backend.compiler);
   String get name() => 'SSA code generator';
@@ skipping 1838 matching lines @@
 
   void visitStaticStore(HStaticStore node) {
     world.registerStaticUse(node.element);
     beginExpression(JSPrecedence.ASSIGNMENT_PRECEDENCE);
     buffer.add(compiler.namer.isolateAccess(node.element));
     buffer.add(' = ');
     use(node.inputs[0], JSPrecedence.ASSIGNMENT_PRECEDENCE);
     endExpression(JSPrecedence.ASSIGNMENT_PRECEDENCE);
   }
 
+  void visitStringConcat(HStringConcat node) {
+    if (isEmptyString(node.left)) {
+      beginExpression(JSPrecedence.CALL_PRECEDENCE);
+      useStringified(node.right, JSPrecedence.EXPRESSION_PRECEDENCE);
+      endExpression(JSPrecedence.CALL_PRECEDENCE);
+   } else if (isEmptyString(node.right)) {
+      beginExpression(JSPrecedence.CALL_PRECEDENCE);
+      useStringified(node.left, JSPrecedence.EXPRESSION_PRECEDENCE);
+      endExpression(JSPrecedence.CALL_PRECEDENCE);
+    } else {
+      JSBinaryOperatorPrecedence operatorPrecedences = JSPrecedence.binary['+'];
+      beginExpression(operatorPrecedences.precedence);
+      useStringified(node.left, operatorPrecedences.left);
+      buffer.add(' + ');
+      // If the right hand side is a string concatenation itself it is
+      // safe to make it left associative.
+      int rightPrecedence = (node.right is HStringConcat)
+          ? JSPrecedence.ADDITIVE_PRECEDENCE
+          : operatorPrecedences.right;
+      useStringified(node.right, rightPrecedence);
+      endExpression(operatorPrecedences.precedence);
+    }
+  }
+
+  bool isEmptyString(HInstruction node) {
+    if (!node.isConstantString()) return false;
+    HConstant constant = node;
+    StringConstant string = constant.constant;
+    return string.value.length == 0;
+  }
+
+  void useStringified(HInstruction node, int precedence) {
+    if (node.isString()) {
+      use(node, precedence);
+    } else {
+      Element convertToString = compiler.findHelper(const SourceString("S"));
+      world.registerStaticUse(convertToString);
+      buffer.add(compiler.namer.isolateAccess(convertToString));
+      buffer.add('(');
+      use(node, JSPrecedence.EXPRESSION_PRECEDENCE);
+      buffer.add(')');
+    }
+  }
+
   void visitLiteralList(HLiteralList node) {
     generateArrayLiteral(node);
   }
 
   void generateArrayLiteral(HLiteralList node) {
     buffer.add('[');
     int len = node.inputs.length;
     for (int i = 0; i < len; i++) {
       if (i != 0) buffer.add(', ');
       use(node.inputs[i], JSPrecedence.ASSIGNMENT_PRECEDENCE);
@@ skipping 760 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endLabeledBlock(HLabeledBlockInformation labeledBlockInfo) {
     if (labeledBlockInfo.body.start.hasGuards()) {
       endBailoutSwitch();
     }
   }
 }