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)
+{
+    var subBlob, subFile;
+    var blobReader = new FileReader();
+    var fileReader = new FileReader();
+    var message = ".slice";
+    if (start === null && end === null) {
+        message += "()";
+        subBlob = blob.slice();
+        subFile = file.slice();
+    } else if (end == undefined) {
+        message += "(" + start + ")";
+        subBlob = blob.slice(start);
+        subFile = file.slice(start);
+    } else {
+        message += "(" + start + ", " + end + ")";
+        subBlob = blob.slice(start, end);
+        subFile = file.slice(start, end);
+    }
+    blobReader.onload = function(event) {
+        window.blobContents = event.target.result;
+        shouldBeEqualToString("blobContents", expectedResult);
+        debug("File " + message);
+        fileReader.readAsText(subFile);
+    };
+    blobReader.onerror = function(event) {
+        testFailed("Blob read error " + message + event.target.error.message);
+        debug("File " + message);
+        fileReader.readAsText(subFile);
+    };
+    fileReader.onload = function(event) {
+        window.fileContents = event.target.result;
+        shouldBeEqualToString("fileContents", expectedResult);
+        runNextTest();
+    };
+    fileReader.onerror = function(event) {
+        testFailed("File read error " + message + event.target.error.message);
+        runNextTest();
+    }
+    debug("Blob " + message);
+    blobReader.readAsText(subBlob);
+}
+
+function runNextTest()
+{
+    if (testIndex >= sliceTestCases.length) {
+        finishJSTest();
+        return;
+    }
+
+    var testCase = sliceTestCases[testIndex];
+    testIndex++;
+    testSlicing(testCase[0], testCase[1], testCase[2]);
+}