Add simple implementation of Array.from() and TypedArray.from()

These will be used for PhantomJS 1.x and browsers that do not support
those methods.

core/util.js

@@ -11,6 +11,32 @@
 
 var Util = {};
 
+var addClassFunc = function (cl, name, func) {
+    if (!cl[name]) {
+        Object.defineProperty(cl, name, { enumerable: false, value: func });
+    }
+};
+
+// TODO(kelleyk): There's probably a better way to do this, but TypedArray isn't
+// directly accessible.
+// N.B.(kelleyk): PhantomJS 1.x does not support Uint8ClampedArray or Float64Array,
+// so those are left out.
+[Array, Int8Array, Uint8Array, Int16Array, Uint16Array, Int32Array, Uint32Array, Float32Array].forEach(
+    function (cls) {
+        var thatCls = cls;
+        addClassFunc(cls, 'from', function (arrayLike,  mapFn, thisArg) {
+            if (typeof(mapFn) !== 'undefined' || typeof(thisArg) !== 'undefined')
+                throw new Error("This version of Array.from() does not support " +
+                                "mapFn or thisArg arguments.");
+    
+            var result = new thatCls(arrayLike.length);
+            for (var i = 0; i < arrayLike.length; ++i)
+                result[i] = arrayLike[i];
+    
+            return result;
+        });
+    });
+
 /*
  * ------------------------------------------------------
  * Namespaced in Util

tests/test.util.js

@@ -7,6 +7,45 @@ var expect = chai.expect;
 describe('Utils', function() {
     "use strict";
 
+    describe('Array class methods', function () {
+
+        var expectTypedArrayEq = function (cls, arr) {
+            var other = cls.from(arr);
+            
+            expect(other).to.be.instanceof(cls);
+
+            // N.B.: We might be tempted to say 'expect(other).to.deep.equal(arr);',
+            // but that would be incorrect in the situation where
+            // 'arr' is not an instance of 'cls'.
+            expect(other.length).to.equal(arr.length);
+            for (var i = 0; i < arr.length; ++i)
+                expect(other[i]).to.equal(arr[i]);
+
+            // TODO: Test deep/shallow copy behavior?
+        };
+
+        describe('Array.from', function () {
+            it('should create a new object with the same type and contents', function () {
+                var arr = [5, 4, 3];
+                expectTypedArrayEq(Array, arr);
+            });
+        });
+        
+        // As a stand-in for all of the TypedArray classes
+        describe('Int32Array.from', function () {
+            it('should create a new object with the same type and contents', function () {
+                var arr = new Int32Array(3);
+                arr[0] = 5; arr[1] = 4; arr[2] = 3;  // want to do this without from(), obviously
+                expectTypedArrayEq(Int32Array, arr);
+            });
+
+            it('should return an Int32Array even if the argument is a different type of array', function () {
+             var arr = [5, 4, 3];
+             expectTypedArrayEq(Int32Array, arr);
+            });
+        });
+    });
+
     describe('logging functions', function () {
         beforeEach(function () {
             sinon.spy(console, 'log');