feat(test): Added mutation testing for scalar values.

This is a port of the tests found in test.cpp

tests/JavaScriptTest.js

@@ -64,7 +64,10 @@ function main() {
 
   fs.writeFileSync('monsterdata_javascript_wire.mon', new Buffer(fbb.asUint8Array()));
 
-  // Test it:
+  // Tests mutation first.  This will verify that we did not trample any other
+  // part of the byte buffer.
+  testMutation(fbb.dataBuffer());
+  
   testBuffer(fbb.dataBuffer());
 
   test64bit();
@@ -74,6 +77,21 @@ function main() {
   console.log('FlatBuffers test: completed successfully');
 }
 
+function testMutation(bb) {
+  var monster = MyGame.Example.Monster.getRootAsMonster(bb);
+
+  monster.mutate_hp(120);
+  assert.strictEqual(monster.hp(), 120);
+
+  monster.mutate_hp(80);
+  assert.strictEqual(monster.hp(), 80);
+
+  var manaRes = monster.mutate_mana(10);
+  assert.strictEqual(manaRes, false);  // Field was NOT present, because default value.
+
+  // TODO: There is not the availability to mutate structs or vectors.
+}
+
 function testBuffer(bb) {
   assert.ok(MyGame.Example.Monster.bufferHasIdentifier(bb));