Fix compilation in webappcreator maven template.

Bug: issue 9054
Change-Id: I3bc957fcf6d55a6b343a2ca4182e7ae96cfd0c1e
Remove usages of '_' as a one-character identifier

Use of '_' as a one-character identifier is deprecated in
Java 9 [1]. In the future it may be used as a keyword [2].

[1] https://bugs.openjdk.java.net/browse/JDK-8061549
[2] http://mail.openjdk.java.net/pipermail/lambda-dev/2013-July/010670.html

Change-Id: Ic9efa1a1f6a741006fe0993a5047196791a4c88c
Review-Link: https://gwt-review.googlesource.com/#/c/10690/
Expand += when there's an implicit cast

A += operator (and similar operators) does an implicit narrowing
cast when the target has less precision. (This is Puzzle #9 in
Java Puzzlers.)

Google is making += where there's an implicit cast a build error
(using an error-prone check). This patch applies an automatic
refactoring to equivalent code.

Change-Id: I58f88a7f5262d8fe3552bb687b47692362eba537