Switch to using ESLint, instead of JSHint, for linting

*Please note that most of the necessary code adjustments were made in PR 7890.*

ESLint has a number of advantageous properties, compared to JSHint. Among those are:
 - The ability to find subtle bugs, thanks to more rules (e.g. PR 7881).
 - Much more customizable in general, and many rules allow fine-tuned behaviour rather than the just the on/off rules in JSHint.
 - Many more rules that can help developers avoid bugs, and a lot of rules that can be used to enforce a consistent coding style. The latter should be particularily useful for new contributors (and reduce the amount of stylistic review comments necessary).
 - The ability to easily specify exactly what rules to use/not to use, as opposed to JSHint which has a default set. *Note:* in future JSHint version some of the rules we depend on will be removed, according to warnings in http://jshint.com/docs/options/, so we wouldn't be able to update without losing lint coverage.
 - More easily disable one, or more, rules temporarily. In JSHint this requires using a numeric code, which isn't very user friendly, whereas in ESLint the rule name is simply used instead.

By default there's no rules enabled in ESLint, but there are some default rule sets available. However, to prevent linting failures if we update ESLint in the future, it seemed easier to just explicitly specify what rules we want.
Obviously this makes the ESLint config file somewhat bigger than the old JSHint config file, but given how rarely that one has been updated over the years I don't think that matters too much.

I've tried, to the best of my ability, to ensure that we enable the same rules for ESLint that we had for JSHint. Furthermore, I've also enabled a number of rules that seemed to make sense, both to catch possible errors *and* various style guide violations.

Despite the ESLint README claiming that it's slower that JSHint, https://github.com/eslint/eslint#how-does-eslint-performance-compare-to-jshint, locally this patch actually reduces the runtime for `gulp` lint (by approximately 20-25%).

A couple of stylistic rules that would have been nice to enable, but where our code currently differs to much to make it feasible:
 - `comma-dangle`, controls trailing commas in Objects and Arrays (among others).
 - `object-curly-spacing`, controls spacing inside of Objects.
 - `spaced-comment`, used to enforce spaces after `//` and `/*. (This is made difficult by the fact that there's still some usage of the old preprocessor left.)

Rules that I indend to look into possibly enabling in follow-ups, if it seems to make sense: `no-else-return`, `no-lonely-if`, `brace-style` with the `allowSingleLine` parameter removed.

Useful links:
 - http://eslint.org/docs/user-guide/configuring
 - http://eslint.org/docs/rules/

src/core/fonts.js

@@ -983,7 +983,7 @@ var Font = (function FontClosure() {
     // Split the sorted codes into ranges.
     var ranges = [];
     var length = codes.length;
-    for (var n = 0; n < length; ) {
+    for (var n = 0; n < length; ) { // eslint-disable-line space-in-parens
       var start = codes[n].fontCharCode;
       var codeIndices = [codes[n].glyphId];
       ++n;

src/core/function.js

@@ -423,7 +423,7 @@ var PDFFunction = (function PDFFunctionClosure() {
         // Compiled function consists of simple expressions such as addition,
         // subtraction, Math.max, and also contains 'var' and 'return'
         // statements. See the generation in the PostScriptCompiler below.
-        /*jshint -W054 */
+        // eslint-disable-next-line no-new-func
         return new Function('src', 'srcOffset', 'dest', 'destOffset', compiled);
       }
 

src/core/jpg.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable no-multi-spaces */
 
 'use strict';
 

src/core/murmurhash3.js

@@ -1,4 +1,3 @@
-
 /* Copyright 2014 Opera Software ASA
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -48,6 +47,7 @@ var MurmurHash3_64 = (function MurmurHash3_64Closure(seed) {
       !PDFJSDev.test('FIREFOX || MOZCENTRAL || CHROME')) {
     // old webkits have issues with non-aligned arrays
     try {
+      // eslint-disable-next-line no-new
       new Uint32Array(new Uint8Array(5).buffer, 0, 1);
     } catch (e) {
       alwaysUseUint32ArrayView = true;

src/core/parser.js

@@ -248,29 +248,29 @@ var Parser = (function ParserClosure() {
           case 0xC1: // SOF1
           case 0xC2: // SOF2
           case 0xC3: // SOF3
-
+            /* falls through */
           case 0xC5: // SOF5
           case 0xC6: // SOF6
           case 0xC7: // SOF7
-
+            /* falls through */
           case 0xC9: // SOF9
           case 0xCA: // SOF10
           case 0xCB: // SOF11
-
+            /* falls through */
           case 0xCD: // SOF13
           case 0xCE: // SOF14
           case 0xCF: // SOF15
-
+            /* falls through */
           case 0xC4: // DHT
           case 0xCC: // DAC
-
+            /* falls through */
           case 0xDA: // SOS
           case 0xDB: // DQT
           case 0xDC: // DNL
           case 0xDD: // DRI
           case 0xDE: // DHP
           case 0xDF: // EXP
-
+            /* falls through */
           case 0xE0: // APP0
           case 0xE1: // APP1
           case 0xE2: // APP2
@@ -287,7 +287,7 @@ var Parser = (function ParserClosure() {
           case 0xED: // APP13
           case 0xEE: // APP14
           case 0xEF: // APP15
-
+            /* falls through */
           case 0xFE: // COM
             // The marker should be followed by the length of the segment.
             markerLength = stream.getUint16();

src/core/pattern.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable no-multi-spaces */
 
 'use strict';
 

src/core/worker.js

@@ -83,14 +83,13 @@ var WorkerTask = (function WorkerTaskClosure() {
 })();
 
 if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('PRODUCTION')) {
-/*jshint -W082 */
 /**
  * Interface that represents PDF data transport. If possible, it allows
  * progressively load entire or fragment of the PDF binary data.
  *
  * @interface
- * */
-function IPDFStream() {}
+ */
+function IPDFStream() {} // eslint-disable-line no-inner-declarations
 IPDFStream.prototype = {
   /**
    * Gets a reader for the entire PDF data.
@@ -118,7 +117,7 @@ IPDFStream.prototype = {
  *
  * @interface
  */
-function IPDFStreamReader() {}
+function IPDFStreamReader() {} // eslint-disable-line no-inner-declarations
 IPDFStreamReader.prototype = {
   /**
    * Gets a promise that is resolved when the headers and other metadata of
@@ -179,7 +178,7 @@ IPDFStreamReader.prototype = {
  *
  * @interface
  */
-function IPDFStreamRangeReader() {}
+function IPDFStreamRangeReader() {} // eslint-disable-line no-inner-declarations
 IPDFStreamRangeReader.prototype = {
   /**
    * Gets ability of the stream to progressively load binary data.

src/display/api.js

@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 /* globals pdfjsFilePath, pdfjsVersion, pdfjsBuild, requirejs, pdfjsLibs,
-           WeakMap */
+           __webpack_require__ */
 
 'use strict';
 

src/display/font_loader.js

@@ -426,7 +426,7 @@ var FontFaceObject = (function FontFaceObjectClosure() {
 
             js += 'c.' + current.cmd + '(' + args + ');\n';
           }
-          /* jshint -W054 */
+          // eslint-disable-next-line no-new-func
           this.compiledGlyphs[character] = new Function('c', 'size', js);
         } else {
           // But fall back on using Function.prototype.apply() if we're

src/display/svg.js

@@ -460,7 +460,7 @@ var SVGGraphics = (function SVGGraphicsClosure() {
     convertOpList: function SVGGraphics_convertOpList(operatorList) {
       var argsArray = operatorList.argsArray;
       var fnArray = operatorList.fnArray;
-      var fnArrayLen  = fnArray.length;
+      var fnArrayLen = fnArray.length;
       var REVOPS = [];
       var opList = [];
 

src/display/webgl.js

@@ -12,7 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* jshint -W043 */
+/* eslint-disable no-multi-str */
 
 'use strict';
 

src/doc_helper.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable strict */
 
 /*
  NOTE: This file is created as a helper to assist with JSDoc html files.

src/pdf.js

@@ -12,7 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* jshint globalstrict: false */
+/* eslint strict: ["error", "function"] */
 /* umdutils ignore */
 
 (function (root, factory) {

src/pdf.worker.entry.js

@@ -12,7 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable strict */
 
 (typeof window !== 'undefined' ? window : {}).pdfjsDistBuildPdfWorker =
   require('./pdf.worker.js');
-

src/shared/fonts_utils.js

@@ -73,7 +73,7 @@ function readCharstringEncoding(aString) {
   var charstringTokens = [];
 
   var count = aString.length;
-  for (var i = 0; i < count; ) {
+  for (var i = 0; i < count; ) { // eslint-disable-line space-in-parens
     var value = aString[i++] | 0;
     var token = null;
 

src/shared/util.js

@@ -620,8 +620,7 @@ function isLittleEndian() {
 // Checks if it's possible to eval JS expressions.
 function isEvalSupported() {
   try {
-    /* jshint evil: true */
-    new Function('');
+    new Function(''); // eslint-disable-line no-new, no-new-func
     return true;
   } catch (e) {
     return false;
@@ -1756,6 +1755,8 @@ if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('MOZCENTRAL')) {
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/ */
 (function checkURLConstructor(scope) {
+  /* eslint-disable yoda */
+
   // feature detect for URL constructor
   var hasWorkingUrl = false;
   try {
@@ -2392,6 +2393,8 @@ if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('MOZCENTRAL')) {
   }
 
   scope.URL = JURL;
+
+  /* eslint-enable yoda */
 })(globalScope);
 }