fix implicit_scope_test and builtin_identifier_test

lib/src/js/nodes.dart

Index: lib/src/js/nodes.dart
diff --git a/lib/src/js/nodes.dart b/lib/src/js/nodes.dart
index 5f099872a7879902bf3b35150a7efaa97883c182..9c64f921ac63375a0ade13ca40505c166204935e 100644
--- a/lib/src/js/nodes.dart
+++ b/lib/src/js/nodes.dart
@@ -278,7 +278,11 @@ abstract class Statement extends Node {
 
 class Block extends Statement {
   final List<Statement> statements;
-  Block(this.statements) {
+
+  /// True to preserve this [Block] for scoping reasons.
+  final bool isScope;
+
+  Block(this.statements, {this.isScope: false}) {
     assert(!statements.any((s) => s is! Statement));
   }
   Block.empty() : this.statements = <Statement>[];