LibJS: Implement basic functionality of Array.from()

The optional 2nd and 3rd arguments are not yet implemented.

This assumes that `this` is the Array constructor and doesn't yet
implement the more general behavior in the ES6 spec that allows
transferring this method to other constructors.

Libraries/LibJS/Tests/builtins/Array/Array.from.js

@@ -0,0 +1,68 @@
+test("length is 1", () => {
+    expect(Array.from).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("empty array", () => {
+        var a = Array.from([]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(0);
+    });
+
+    test("empty string", () => {
+        var a = Array.from("");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(0);
+    });
+
+    test("non-empty array", () => {
+        var a = Array.from([5, 8, 1]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(3);
+        expect(a[0]).toBe(5);
+        expect(a[1]).toBe(8);
+        expect(a[2]).toBe(1);
+    });
+
+    test("non-empty string", () => {
+        var a = Array.from("what");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(4);
+        expect(a[0]).toBe("w");
+        expect(a[1]).toBe("h");
+        expect(a[2]).toBe("a");
+        expect(a[3]).toBe("t");
+    });
+
+    test("shallow array copy", () => {
+        var a = [1, 2, 3];
+        var b = Array.from([a]);
+        expect(b instanceof Array).toBeTrue();
+        expect(b).toHaveLength(1);
+        b[0][0] = 4;
+        expect(a[0]).toBe(4);
+    });
+
+    test("from iterator", () => {
+        function rangeIterator(begin, end) {
+            return {
+                [Symbol.iterator]() {
+                    let value = begin - 1;
+                    return {
+                        next() {
+                            if (value < end)
+                                value += 1;
+                            return { value: value, done: value >= end };
+                        },
+                    };
+                },
+            };
+        }
+
+        var a = Array.from(rangeIterator(8, 10));
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(2);
+        expect(a[0]).toBe(8);
+        expect(a[1]).toBe(9);
+    });
+});