dart2js: Move lowering of identical(a, b) from builder to optimizer

pkg/compiler/lib/src/ssa/optimize.dart

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index d84aaf356ca95218c7d4b03a5940ccc3a1107a00..b7b0c0ac5b2bc9e668d10487bfbfc1021d9043d6 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -1005,7 +1005,13 @@ class SsaInstructionSimplifier extends HBaseVisitor
     propagateConstantValueToUses(node);
     MemberEntity element = node.element;
 
-    if (element == backend.helpers.checkConcurrentModificationError) {
+    if (element == compiler.commonElements.identicalFunction) {
+      if (node.inputs.length == 2) {
+        return new HIdentity(node.inputs[0], node.inputs[1], null,
+            closedWorld.commonMasks.boolType)
+          ..sourceInformation = node.sourceInformation;
+      }
+    } else if (element == backend.helpers.checkConcurrentModificationError) {
       if (node.inputs.length == 2) {
         HInstruction firstArgument = node.inputs[0];
         if (firstArgument is HConstant) {