refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with functions which work similarily but aren't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

spec/core/asymmetric_equality/ArrayWithExactContentsSpec.js

@@ -48,8 +48,8 @@ describe('ArrayWithExactContents', function() {
       if (
         typeof a == 'string' &&
         typeof b == 'string' &&
-        a.substr(0, 3) == 'foo' &&
-        b.substr(0, 3) == 'foo'
+        a.slice(0, 3) == 'foo' &&
+        b.slice(0, 3) == 'foo'
       ) {
         return true;
       }