LibJS: Add Proxy objects

Includes all traps except the following: [[Call]], [[Construct]],
[[OwnPropertyKeys]].

An important implication of this commit is that any call to any virtual
Object method has the potential to throw an exception. These methods
were not checked in this commit -- a future commit will have to protect
these various method calls throughout the codebase.

Libraries/LibJS/Tests/Proxy.handler-defineProperty.js

@@ -0,0 +1,107 @@
+load("test-common.js");
+
+try {
+    let p = new Proxy({}, { defineProperty: null });
+    assert(Object.defineProperty(p, "foo", {}) === p);
+    p = new Proxy({}, { defineProperty: undefined });
+    assert(Object.defineProperty(p, "foo", {}) === p);
+    p = new Proxy({}, {});
+    assert(Object.defineProperty(p, "foo", {}) == p);
+
+    let o = {};
+    p = new Proxy(o, {
+        defineProperty(target, name, descriptor) {
+            assert(target === o);
+            assert(name === "foo");
+            assert(descriptor.configurable === true);
+            assert(descriptor.enumerable === undefined);
+            assert(descriptor.writable === true);
+            assert(descriptor.value === 10);
+            assert(descriptor.get === undefined);
+            assert(descriptor.set === undefined);
+            return true;
+        },
+    });
+
+    Object.defineProperty(p, "foo", { configurable: true, writable: true, value: 10 });
+
+    p = new Proxy(o, {
+        defineProperty(target, name, descriptor) {
+            if (target[name] === undefined)
+                Object.defineProperty(target, name, descriptor);
+            return true;
+        },
+    });
+
+    Object.defineProperty(p, "foo", { value: 10, enumerable: true, configurable: false, writable: true });
+    let d = Object.getOwnPropertyDescriptor(p, "foo");
+    assert(d.enumerable === true);
+    assert(d.configurable === false);
+    assert(d.writable === true);
+    assert(d.value === 10);
+    assert(d.get === undefined);
+    assert(d.set === undefined);
+
+    Object.defineProperty(p, "foo", { value: 20, enumerable: true, configurable: false, writable: true });
+    d = Object.getOwnPropertyDescriptor(p, "foo");
+    assert(d.enumerable === true);
+    assert(d.configurable === false);
+    assert(d.writable === true);
+    assert(d.value === 10);
+    assert(d.get === undefined);
+    assert(d.set === undefined);
+
+
+    // Invariants
+
+    p = new Proxy({}, {
+        defineProperty() { return false; }
+    });
+
+    assertThrowsError(() => {
+        Object.defineProperty(p, "foo", {});
+    }, {
+        error: TypeError,
+        message: "Proxy handler's defineProperty method returned false",
+    });
+
+    o = {};
+    Object.preventExtensions(o);
+    p = new Proxy(o, {
+        defineProperty() {
+            return true;
+        }
+    });
+    assertThrowsError(() => {
+        Object.defineProperty(p, "foo", {});
+    }, {
+        error: TypeError,
+        message: "Proxy handler's defineProperty trap violates invariant: a property cannot be reported as being defined if the property does not exist on the target and the target is non-extensible",
+    });
+
+    o = {};
+    Object.defineProperty(o, "foo", { value: 10, configurable: true });
+    p = new Proxy(o, {
+        defineProperty() {
+            return true;
+        },
+    });
+
+    assertThrowsError(() => {
+        Object.defineProperty(p, "bar", { value: 6, configurable: false });
+    }, {
+        error: TypeError,
+        message: "Proxy handler's defineProperty trap violates invariant: a property cannot be defined as non-configurable if it does not already exist on the target object",
+    });
+
+    assertThrowsError(() => {
+        Object.defineProperty(p, "foo", { value: 6, configurable: false });
+    }, {
+        error: TypeError,
+        message: "Proxy handler's defineProperty trap violates invariant: a property cannot be defined as non-configurable if it already exists on the target object as a configurable property",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}