fix strong mode messages in pkg/dev_compiler

pkg/dev_compiler/lib/src/compiler/code_generator.dart

Index: pkg/dev_compiler/lib/src/compiler/code_generator.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/code_generator.dart b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
index bc48e342fb7fc1df283a2dfa272b89b5b8d2f36f..783e7da39cf254546081c1448d3c88dbd217a656 100644
--- a/pkg/dev_compiler/lib/src/compiler/code_generator.dart
+++ b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
@@ -5095,8 +5095,7 @@ class CodeGenerator extends GeneralizingAstVisitor
   _visit(AstNode node) {
     if (node == null) return null;
     var result = node.accept(this);
-    if (result is JS.Node) result = annotate(result, node);
-    return result;
+    return result is JS.Node ? annotate(result, node) : result;
   }
 
   List/*<T>*/ _visitList/*<T extends AstNode>*/(Iterable/*<T>*/ nodes) {