j2cl:watch should require that the compile phase runs before it

[niloc132]

When running mvn j2cl:watch from the command line, if there were other bound goals in the pom, they might not run if j2cl:watch doesn't ensure that all generated sources are always available. Side effect from this could include missing generated sources, or source directories not added to the maven project model.

To fix this, j2cl:watch should insist that the compile phase completes before it starts.