Fixed typo

Fix mac build error

is => it

Also added simple (in-process) test.

Change-Id: I38580d554dd52f590e3396ec4846e07546dcf07d
Tested: on Linux.

Added new constants

Add new constants in Constants.java

All single ints int FlatBufferBuilder.java were replaced by constants
from Constants.java

pulling methods inside the class... so as to share wrapInNamespace later

This is useful because many JSON generators will sort the fields,
cause X_type to follow X.

Change-Id: I00ef3ac05418224fc05aee93e6b3b3597e73ffe3
Tested: on Linux.
Bug: 29221752

Also fixed most codegenerators using the wrong namespace when multiple
namespace were used in a file, with some files not being generated.

Change-Id: Ib42969221239d7244e431cbd667ef69200fc415f
Tested: on Linux.
Bug: 29338474

Fix test using compat.binary_types (from March).