dart2js cps: Introduce some built-in operators in type propagation.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 13ccd0c9589ebb0bbec3ec47679ea3249fa0f8e0..deaa2ed8434066d9ba0f2790629fc7d0be4971b3 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -583,5 +583,13 @@ class Builder implements cps_ir.Visitor<Node> {
         node.sourceInformation);
     return new ExpressionStatement(setStatic, visit(node.body));
   }
+
+  Expression visitApplyBuiltinOperator(cps_ir.ApplyBuiltinOperator node) {
+    if (node.operator == BuiltinOperator.IsFalsy) {
+      return new Not(getVariableUse(node.arguments.single));
+    }
+    return new ApplyBuiltinOperator(node.operator,
+                                    translateArguments(node.arguments));
+  }
 }