Implement File constructor.

LayoutTests/fast/files/resources/blob-slice-common.js

+var blob, file; // Populated by runTests() in individual tests.
+var sliceParams = []; // Populated by individual tests.
+var testIndex = 0;
+
+function testSlicing(start, end, expectedResult, blob, doneCallback)
+{
+    var blobClass = blob.constructor.name;
+    var sliced;
+    var reader = new FileReader();
+    var message = ".slice";
+    if (start === null && end === null) {
+        message += "()";
+        sliced = blob.slice();
+    } else if (end == undefined) {
+        message += "(" + start + ")";
+        sliced = blob.slice(start);
+    } else {
+        message += "(" + start + ", " + end + ")";
+        sliced = blob.slice(start, end);
+    }
+    reader.onloadend = function(event) {
+        var error = event.target.error;
+        if (error) {
+            testFailed("File read error " + message + error);
+            doneCallback();
+            return;
+        }
+        var blobContentsVar = blobClass.toLowerCase() + "Contents";
+        window[blobContentsVar] = event.target.result;
+        shouldBeEqualToString(blobContentsVar, expectedResult);
+        doneCallback();
+    };
+    debug(blobClass + " " + message);
+    reader.readAsText(sliced);
+}
+
+function runNextTest()
+{
+    if (testIndex >= sliceTestCases.length) {
+        finishJSTest();
+        return;
+    }
+
+    var testCase = sliceTestCases[testIndex];
+    testIndex++;
+    testSlicing(testCase[0], testCase[1], testCase[2], blob, function() {
+        testSlicing(testCase[0], testCase[1], testCase[2], file, runNextTest);
+    });
+}