support asynchronous arrow functions (#4530)

lib/ast.js

@@ -567,8 +567,20 @@ var AST_Accessor = DEFNODE("Accessor", null, {
     },
 }, AST_Lambda);
 
+function is_arrow(node) {
+    return node instanceof AST_AsyncArrow || node instanceof AST_Arrow;
+}
+
 function is_function(node) {
-    return node instanceof AST_Arrow || node instanceof AST_AsyncFunction || node instanceof AST_Function;
+    return is_arrow(node) || node instanceof AST_AsyncFunction || node instanceof AST_Function;
+}
+
+function walk_lambda(node, tw) {
+    if (is_arrow(node) && node.value) {
+        node.value.walk(tw);
+    } else {
+        walk_body(node, tw);
+    }
 }
 
 var AST_Arrow = DEFNODE("Arrow", "inlined value", {
@@ -601,9 +613,38 @@ var AST_Arrow = DEFNODE("Arrow", "inlined value", {
 }, AST_Lambda);
 
 function is_async(node) {
-    return node instanceof AST_AsyncDefun || node instanceof AST_AsyncFunction;
+    return node instanceof AST_AsyncArrow || node instanceof AST_AsyncDefun || node instanceof AST_AsyncFunction;
 }
 
+var AST_AsyncArrow = DEFNODE("AsyncArrow", "inlined value", {
+    $documentation: "An asynchronous arrow function expression",
+    $propdoc: {
+        value: "[AST_Node?] simple return expression, or null if using function body.",
+    },
+    walk: function(visitor) {
+        var node = this;
+        visitor.visit(node, function() {
+            node.argnames.forEach(function(argname) {
+                argname.walk(visitor);
+            });
+            if (node.rest) node.rest.walk(visitor);
+            if (node.value) {
+                node.value.walk(visitor);
+            } else {
+                walk_body(node, visitor);
+            }
+        });
+    },
+    _validate: function() {
+        if (this.name != null) throw new Error("name must be null");
+        if (this.uses_arguments) throw new Error("uses_arguments must be false");
+        if (this.value != null) {
+            must_be_expression(this, "value");
+            if (this.body.length) throw new Error("body must be empty if value exists");
+        }
+    },
+}, AST_Lambda);
+
 var AST_AsyncFunction = DEFNODE("AsyncFunction", "inlined name", {
     $documentation: "An asynchronous function expression",
     $propdoc: {