LibJS: Add symbol objects

This commit adds the following classes: SymbolObject, SymbolConstructor,
SymbolPrototype, and Symbol. This commit does not introduce any
new functionality to the Object class, so they cannot be used as
property keys in objects.

Libraries/LibJS/Tests/Symbol.for.js

@@ -0,0 +1,30 @@
+load("test-common.js")
+
+try {
+    const localSym = Symbol("foo");
+    const globalSym = Symbol.for("foo");
+
+    assert(localSym !== globalSym);
+    assert(localSym !== Symbol("foo"));
+    assert(globalSym !== Symbol("foo"));
+    assert(globalSym === Symbol.for("foo"));
+    assert(localSym.toString() === "Symbol(foo)");
+    assert(globalSym.toString() === "Symbol(foo)");
+
+    assert(Symbol.for(1).description === "1");
+    assert(Symbol.for(true).description === "true");
+    assert(Symbol.for({}).description === "[object Object]");
+    assert(Symbol.for().description === "undefined");
+    assert(Symbol.for(null).description === "null");
+
+    assertThrowsError(() => {
+        Symbol.for(Symbol());
+    }, {
+        error: TypeError,
+        message: "Can't convert symbol to string",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/Symbol.js

@@ -0,0 +1,26 @@
+load("test-common.js")
+
+try {
+    const s1 = Symbol("foo");
+    const s2 = Symbol("foo");
+
+    assert(s1 !== s2);
+    assert(s1.description === "foo");
+    assert(s2.description === "foo");
+
+    s1.description = "bar";
+    assert(s1.description === "foo");
+
+    assert(typeof s1 === "symbol");
+
+    assertThrowsError(() => {
+        Symbol(Symbol('foo'));
+    }, {
+        error: TypeError,
+        message: "Can't convert symbol to string"
+    })
+    
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/Symbol.keyFor.js

@@ -0,0 +1,31 @@
+load("test-common.js")
+
+try {
+    const localSym = Symbol("foo");
+    const globalSym = Symbol.for("foo");
+
+    assert(Symbol.keyFor(localSym) === undefined);
+    assert(Symbol.keyFor(globalSym) === "foo");
+
+    const testThrows = (value, str) => {
+        assertThrowsError(() => {
+            Symbol.keyFor(value);
+        }, {
+            error: TypeError,
+            message: str + " is not a symbol",
+        });
+    };
+
+    testThrows(1, "1");
+    testThrows(null, "null");
+    testThrows(undefined, "undefined");
+    testThrows([], "[object Array]");
+    testThrows({}, "[object Object]");
+    testThrows(true, "true");
+    testThrows("foobar", "foobar");
+    testThrows(function(){}, "[object ScriptFunction]"); // FIXME: Better function stringification
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/Symbol.prototype.toString.js

@@ -0,0 +1,28 @@
+load("test-common.js")
+
+try {
+    const s1 = Symbol("foo");
+    const s2 = Symbol.for("bar");
+
+    assert(s1.toString() === "Symbol(foo)");
+    assert(s2.toString() === "Symbol(bar)");
+    
+    assertThrowsError(() => {
+        s1 + "";
+    }, {
+        error: TypeError,
+        message: "Can't convert symbol to string",
+    });
+    
+    // FIXME: Uncomment when this doesn't assert
+    // assertThrowsError(() => {
+    //     s1 + 1;
+    // }, {
+    //     error: TypeError,
+    //     message: "Can't convert symbol to number",
+    // });
+    
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+} 

Libraries/LibJS/Tests/Symbol.prototype.valueOf.js

@@ -0,0 +1,22 @@
+load("test-common.js");
+
+try {
+    let local = Symbol('foo');
+    let global = Symbol.for('foo');
+    assert(local.valueOf() === local);
+    assert(global.valueOf() === global);
+
+    assert(Symbol.prototype.valueOf.call(local) === local);
+    assert(Symbol.prototype.valueOf.call(global) === global);
+
+    assertThrowsError(() => {
+        Symbol.prototype.valueOf.call("foo");
+    }, {
+        error: TypeError,
+        message: "object must be of type Symbol"
+    });
+
+    console.log("PASS");
+} catch (err) {
+    console.log("FAIL: " + err);
+}