Move MessageHandler into a separate src/shared/message_handler.js file

The `MessageHandler` itself, and its assorted helper functions, are currently the single largest[1] piece of code in the `src/shared/util.js` file. By moving this code into its own file, `src/shared/util.js` thus becomes smaller and more manageable.

test/unit/clitests.json

@@ -20,6 +20,7 @@
     "evaluator_spec.js",
     "fonts_spec.js",
     "function_spec.js",
+    "message_handler_spec.js",
     "metadata_spec.js",
     "murmurhash3_spec.js",
     "network_utils_spec.js",
@@ -31,7 +32,6 @@
     "type1_parser_spec.js",
     "ui_utils_spec.js",
     "unicode_spec.js",
-    "util_spec.js",
-    "util_stream_spec.js"
+    "util_spec.js"
   ]
 }

test/unit/jasmine-boot.js

@@ -62,6 +62,7 @@ function initializePDFJS(callback) {
     'pdfjs-test/unit/evaluator_spec',
     'pdfjs-test/unit/fonts_spec',
     'pdfjs-test/unit/function_spec',
+    'pdfjs-test/unit/message_handler_spec',
     'pdfjs-test/unit/metadata_spec',
     'pdfjs-test/unit/murmurhash3_spec',
     'pdfjs-test/unit/network_spec',
@@ -74,7 +75,6 @@ function initializePDFJS(callback) {
     'pdfjs-test/unit/ui_utils_spec',
     'pdfjs-test/unit/unicode_spec',
     'pdfjs-test/unit/util_spec',
-    'pdfjs-test/unit/util_stream_spec',
   ].map(function (moduleName) {
     return SystemJS.import(moduleName);
   })).then(function(modules) {

test/unit/message_handler_spec.js

@@ -13,9 +13,10 @@
  * limitations under the License.
  */
 
-import { createPromiseCapability, MessageHandler } from '../../src/shared/util';
+import { createPromiseCapability } from '../../src/shared/util';
+import { MessageHandler } from '../../src/shared/message_handler';
 
-describe('util_stream', function () {
+describe('message_handler', function () {
   // Temporary fake port for sending messages between main and worker.
   class FakePort {
     constructor() {