tree-ir: Treat getInterceptor and identical as effectively constant.

pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart

Index: pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart b/pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart
index 755e4b46e680931e27edf5ac3a0f398420388a76..3499f3861309ddfffc7a51b3d1fd2d4276c81f20 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.dart
@@ -37,7 +37,7 @@ class JsTreeBuilder extends Builder {
         identicalFunction,
         identicalSelector,
         <Expression>[getVariableUse(node.left),
-                     getVariableUse(node.right)]);
+                     getVariableUse(node.right)])..isEffectivelyConstant = true;
   }
 
   Expression visitInterceptor(cps_ir.Interceptor node) {
@@ -48,7 +48,7 @@ class JsTreeBuilder extends Builder {
     return new InvokeStatic(
         getInterceptor,
         selector,
-        <Expression>[getVariableUse(node.input)]);
+        <Expression>[getVariableUse(node.input)])..isEffectivelyConstant = true;
   }
 
   Expression visitGetField(cps_ir.GetField node) {