Sorry for resurfacing this old thread. I did try to compile my simple project [1] against Java 7 SDK and it works for me. It also works if I configure the maven build of my project to use ECJ. I'm not sure why it fails in our Eclipse builds with tycho but not with a simple maven project that targets to Java 7 and is compiled against a Java 7 libraries.
Note that if I try to compile against one of the new methods in FrameworkUtil that returns an Optional it then fails as expected. But compiling against old methods with no Optional in their signature compile fine when also compiling against the Java 7 SDK libraries.
This only concerns me because it would appear Tycho will aggressively force all bundles that use the existing FrameworkUtil.getBundle(Object) method to move up to Java 8 in order to compile. Some will see this as a good thing, nobody should be targeting Java 7 at all anymore! But others will see this as an unnecessary change they have to make just to work around some build issue.
In the end this is not really an Eclipse Project issue, we can easily move all our bundles to Java 8 or later. I only worry about consumers of the new release that may run into this when they start compiling against our next release.