Convert a == to a builtin identity check if the receiver type can only hit Object==.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 20306)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -655,6 +655,7 @@
   Element jsStringSplit;
   Element jsStringConcat;
   Element jsStringToString;
+  Element objectEquals;
 
   ClassElement typeLiteralClass;
   ClassElement mapLiteralClass;
@@ -897,6 +898,9 @@
     constMapLiteralClass =
         compiler.findHelper(const SourceString('ConstantMap'));
 
+    objectEquals = compiler.lookupElementIn(
+        compiler.objectClass, const SourceString('=='));
+
     specialOperatorEqClasses
         ..add(jsNullClass)
         ..add(jsNumberClass);