Don't create new nodes if we don't need to.

lib/compiler/implementation/ssa/optimize.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.
 
 interface OptimizationPhase {
   String get name();
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 661 matching lines @@
   }
 
   void visitIndex(HIndex node) {
     if (!node.receiver.isIndexablePrimitive()) return;
     HInstruction index = node.index;
     if (index is HBoundsCheck) return;
     if (!node.index.isInteger()) {
       index = insertIntegerCheck(node, index);
     }
     index = insertBoundsCheck(node, node.receiver, index);
-    HIndex newInstruction = new HIndex(node.target, node.receiver, index);
-    node.block.addBefore(node, newInstruction);
-    node.block.rewrite(node, newInstruction);
-    node.block.remove(node);
+    node.changeUse(node.index, index);
   }
 
   void visitIndexAssign(HIndexAssign node) {
     if (!node.receiver.isMutableArray()) return;
     HInstruction index = node.index;
     if (index is HBoundsCheck) return;
     if (!node.index.isInteger()) {
       index = insertIntegerCheck(node, index);
     }
     index = insertBoundsCheck(node, node.receiver, index);
-    HIndexAssign newInstruction =
-        new HIndexAssign(node.target, node.receiver, index, node.value);
-    node.block.addBefore(node, newInstruction);
-    node.block.rewrite(node, newInstruction);
-    node.block.remove(node);
+    node.changeUse(node.index, index);
   }
 }
 
 class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
   final String name = "SsaDeadCodeEliminator";
 
   static bool isDeadCode(HInstruction instruction) {
     return !instruction.hasSideEffects()
            && instruction.usedBy.isEmpty()
            && instruction is !HCheck
@@ skipping 595 matching lines @@
       // this type for the field is still a strong signal
       // indicating the expected type of the field.
       field.propagatedType = type;
     } else {
       // If there are no invoked setters we know the type of
       // this field for sure.
       field.guaranteedType = type;
     }
   }
 }