[json] handle proxies in BasicJsonSerializer.

test/mjsunit/es6/proxies-json.js

Index: test/mjsunit/es6/proxies-json.js
diff --git a/test/mjsunit/es6/proxies-json.js b/test/mjsunit/es6/proxies-json.js
index 03dfabecea100ae71ad4d81b1b3a1e94dcd002ba..6b40e3ee7d8d68796be0e8a22f28a888004ac06e 100644
--- a/test/mjsunit/es6/proxies-json.js
+++ b/test/mjsunit/es6/proxies-json.js
@@ -507,3 +507,56 @@ for (var i in log) assertSame(target, log[i][1]);
 assertEquals(["get", target, "length", proxy], log[0]);
 assertEquals(["get", target, "0", proxy], log[1]);
 assertEquals(["deleteProperty", target, "0"], log[2]);
+
+proxy = new Proxy([], {
+  get: function(target, property) {
+    if (property == "length") return 7;
+    return 0;
+  },
+});
+assertEquals('[[0,0,0,0,0,0,0]]', JSON.stringify([proxy]));
+
+proxy = new Proxy([], {
+  get: function(target, property) {
+    if (property == "length") return 1E40;
+    return 0;
+  },
+});
+assertThrows(() => JSON.stringify([proxy]), RangeError);
+
+log = [];
+proxy = new Proxy({}, {
+  ownKeys: function() {
+    log.push("ownKeys");
+    return ["0", "a", "b"];
+  },
+  get: function(target, property) {
+    log.push("get " + property);
+    return property.toUpperCase();
+  },
+  getOwnPropertyDescriptor: function(target, property) {
+    log.push("descriptor " + property);
+    return {enumerable: true, configurable: true};
+  },
+  isExtensible: assertUnreachable,
+  has: assertUnreachable,
+  getPrototypeOf: assertUnreachable,
+  setPrototypeOf: assertUnreachable,
+  preventExtensions: assertUnreachable,
+  setPrototypeOf: assertUnreachable,
+  defineProperty: assertUnreachable,
+  set: assertUnreachable,
+  deleteProperty: assertUnreachable,
+  apply: assertUnreachable,
+  construct: assertUnreachable,
+});
+
+assertEquals('[{"0":"0","a":"A","b":"B"}]', JSON.stringify([proxy]));
+assertEquals(['get toJSON',
+              'ownKeys',
+              'descriptor 0',
+              'descriptor a',
+              'descriptor b',
+              'get 0',
+              'get a',
+              'get b'], log);