Make sure to bind a method to its receiver if it is stored in a temporary.

pkg/js_ast/lib/src/nodes.dart

Index: pkg/js_ast/lib/src/nodes.dart
diff --git a/pkg/js_ast/lib/src/nodes.dart b/pkg/js_ast/lib/src/nodes.dart
index 4fe35e53d7b86726e598b3a980759ca9c928ddd0..18db66c67fc8e1c00d375d660cf0b8e16c579140 100644
--- a/pkg/js_ast/lib/src/nodes.dart
+++ b/pkg/js_ast/lib/src/nodes.dart
@@ -243,7 +243,9 @@ class Block extends Statement {
 
 class ExpressionStatement extends Statement {
   final Expression expression;
-  ExpressionStatement(this.expression);
+  ExpressionStatement(this.expression) {
+    assert(this.expression != null);
+  }
 
   accept(NodeVisitor visitor) => visitor.visitExpressionStatement(this);
   void visitChildren(NodeVisitor visitor) { expression.accept(visitor); }