While we await official Java 1.8 support, I've made a github fork that contains what I believe are the required changes - these require the Java 8 patches from here first: https://wiki.eclipse.org/JDT_Core/Java8
To install the 1.8 facet support (assuming that you're running 4.3.1 or 4.3.2, and have write access to the main .../eclipse/plugins dir):
- git clone https://github.com/stevestorey/webtools.common.fproj
- git checkout java8-support
- Import the various projects in this feature set into your Eclipse IDE (can be on a new workspace). Specifically you need org.eclipse.wst.common.project.facet.core and org.eclipse.wst.common.project.facet.core
- Right click on the org.eclipse.jst.common.project.facet.core project and choose "Export" and then "Plug-in development\Deployable plugins and fragments"
- From the Export dialog, select only the org.eclipse.jst.common.project.facet.core plugin, and choose the bottom "Install into host" radio button, and choose a suitable path (I believe I went with the default, but I don't think it particularly matters)
- Clicking Finish will build the plugin, and then kick off the installation into your Eclipse environment.
After restarting Eclipse, you should find that 1.8 is now selectable as a Java facet :) (and it also works with M2E enabled projects that specify source/target of 1.8).
Note the instruction from the standard 1.8 patch instructions that the installed JDK /must/ be called "JavaSE-1.8" within the Eclipse properties.
Also note that if/when you update Eclipse (e.g. to upgrade to 4.3.2), you will probably need to re-install the patched version again as the update will put the "official" version of the plugin back in.