dart2js cps: specialize isEmpty and isNotEmpty

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 19e8c87d6a79f8d57f733e12d32ac049aa7c36ae..aa26e319231398246ef0cd811da339ff54222879 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -1388,6 +1388,28 @@ class TransformingVisitor extends DeepRecursiveVisitor {
         assert(node.hasNoUses);
         return cps;
 
+      case 'isEmpty':
+        if (!node.selector.isGetter) return null;
+        CpsFragment cps = new CpsFragment(node.sourceInformation);
+        Primitive length = cps.letPrim(new GetLength(receiver));
+        Constant zero = cps.makeZero();
+        ApplyBuiltinOperator op = cps.applyBuiltin(BuiltinOperator.StrictEq,
+                                                   [length, zero]);
+        node.replaceUsesWith(op);
+        op.hint = node.hint;
+        return cps;
+
+      case 'isNotEmpty':
+        if (!node.selector.isGetter) return null;
+        CpsFragment cps = new CpsFragment(node.sourceInformation);
+        Primitive length = cps.letPrim(new GetLength(receiver));
+        Constant zero = cps.makeZero();
+        ApplyBuiltinOperator op = cps.applyBuiltin(BuiltinOperator.StrictNeq,
+                                                   [length, zero]);
+        node.replaceUsesWith(op);
+        op.hint = node.hint;
+        return cps;
+
       default:
         return null;
     }