Restore the btoa/atob polyfills for Node.js

These were removed in PR 9170, since they were unused in the browsers that we'll support in PDF.js version `2.0`.
However looking at the output of Travis, where a subset of the unit-tests are run using Node.js, there's warnings about `btoa` being undefined. This doesn't appear to cause any errors, which probably explains why we didn't notice this before (despite PR 9201).

test/unit/api_spec.js

@@ -17,9 +17,8 @@ import {
   buildGetDocumentParams, NodeFileReaderFactory, TEST_PDFS_PATH
 } from './test_utils';
 import {
-  createPromiseCapability, FontType, InvalidPDFException, isNodeJS,
-  MissingPDFException, PasswordException, PasswordResponses, StreamType,
-  stringToBytes
+  createPromiseCapability, FontType, InvalidPDFException, MissingPDFException,
+  PasswordException, PasswordResponses, StreamType, stringToBytes
 } from '../../src/shared/util';
 import {
   DOMCanvasFactory, RenderingCancelledException
@@ -27,6 +26,7 @@ import {
 import {
   getDocument, PDFDocumentProxy, PDFPageProxy
 } from '../../src/display/api';
+import isNodeJS from '../../src/shared/is_node';
 import { PDFJS } from '../../src/display/global';
 
 describe('api', function() {

test/unit/cmap_spec.js

@@ -15,7 +15,7 @@
 
 import { CMap, CMapFactory, IdentityCMap } from '../../src/core/cmap';
 import { DOMCMapReaderFactory } from '../../src/display/dom_utils';
-import { isNodeJS } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 import { Name } from '../../src/core/primitives';
 import { NodeCMapReaderFactory } from './test_utils';
 import { StringStream } from '../../src/core/stream';

test/unit/custom_spec.js

@@ -16,7 +16,7 @@
 import { buildGetDocumentParams } from './test_utils';
 import { DOMCanvasFactory } from '../../src/display/dom_utils';
 import { getDocument } from '../../src/display/api';
-import { isNodeJS } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 
 function getTopLeftPixel(canvasContext) {
   let imgData = canvasContext.getImageData(0, 0, 1, 1);

test/unit/display_svg_spec.js

@@ -14,10 +14,11 @@
  */
 /* globals __non_webpack_require__ */
 
-import { isNodeJS, NativeImageDecoding } from '../../src/shared/util';
 import { setStubs, unsetStubs } from '../../examples/node/domstubs';
 import { buildGetDocumentParams } from './test_utils';
 import { getDocument } from '../../src/display/api';
+import isNodeJS from '../../src/shared/is_node';
+import { NativeImageDecoding } from '../../src/shared/util';
 import { SVGGraphics } from '../../src/display/svg';
 
 const XLINK_NS = 'http://www.w3.org/1999/xlink';

test/unit/dom_utils_spec.js

@@ -16,7 +16,7 @@
 import {
   DOMSVGFactory, getFilenameFromUrl, isExternalLinkTargetSet, LinkTarget
 } from '../../src/display/dom_utils';
-import { isNodeJS } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 import { PDFJS } from '../../src/display/global';
 
 describe('dom_utils', function() {

test/unit/node_stream_spec.js

@@ -14,7 +14,8 @@
  */
 /* globals __non_webpack_require__ */
 
-import { assert, isNodeJS } from '../../src/shared/util';
+import { assert } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 import { PDFNodeStream } from '../../src/display/node_stream';
 
 // Make sure that we only running this script is Node.js environments.

test/unit/test_utils.js

@@ -13,7 +13,8 @@
  * limitations under the License.
  */
 
-import { CMapCompressionType, isNodeJS } from '../../src/shared/util';
+import { CMapCompressionType } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 import { isRef } from '../../src/core/primitives';
 
 class NodeFileReaderFactory {

test/unit/ui_utils_spec.js

@@ -17,7 +17,8 @@ import {
   binarySearchFirstItem, EventBus, getPDFFileNameFromURL, isValidRotation,
   waitOnEventOrTimeout, WaitOnType
 } from '../../web/ui_utils';
-import { createObjectURL, isNodeJS } from '../../src/shared/util';
+import { createObjectURL } from '../../src/shared/util';
+import isNodeJS from '../../src/shared/is_node';
 
 describe('ui_utils', function() {
   describe('binary search', function() {