If you're using 2.7.0-SNAPSHOT, your build might start failing as of today as we started unbundling dependencies from gwt-dev.
If you get this error, be sure to also update your gwt-maven-plugin to the latest 2.7.0-SNAPSHOT too.
If that doesn't work, add gwt-dev or (better) gwt-codeserver as a project dependency (and report the problem here). This is likely to become the preferred way to use GWT in the future, and the gwt-maven-plugin might stop automatically adding GWT deps to the classpath in gwt:compile or gwt:run in a future version (probably not 2.7 though): as we'll unbundle more dependencies from gwt-dev (and gwt-user), you might want to gain control over those dependencies (e.g. using a newer version of Guava, or ASM, etc.) and this is not possible with plugin dependencies as it's done today within the gwt-maven-plugin.
Also noteworthy: that same change also turned com.google.gwt:gwt into a BOM (bill of material); that means that instead of including the <version> in all your GWT dependencies you can now declare com.google.gwt:gwt into your dependencyManagement section with <scope>import</scope>. As a bonus, it should ensure transitive dependencies to GWT will all use the same version, so you'll have fewer risks of conflicts.
I'll soon update the GWT samples, WebAppCreator templates and various archetypes to use those new "best practices".