Use specific messages for const constructor with body.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index cfea55ab65778d5937a52792dcfeb99ef8969315..fcd360a5e8cc92f860bc7a075329d1dc3239f835 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -308,8 +308,8 @@ class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
     if (constructor.isSynthesized) return null;
     ast.FunctionExpression node = constructor.node;
     // If we know the body doesn't have any code, we don't generate it.
-    if (!node.hasBody()) return null;
-    if (node.hasEmptyBody()) return null;
+    if (!node.hasBody) return null;
+    if (node.hasEmptyBody) return null;
     ClassElement classElement = constructor.enclosingClass;
     ConstructorBodyElement bodyElement;
     classElement.forEachBackendMember((Element backendMember) {