Eclipse plugin - Caused by: java.lang.ClassNotFoundException : clazz cannot be found by - eclipse

Question

I've successfully created an Eclipse-plugin to which I'm adding a command in order to wrap some text with <xsl:text>.
My environment is Win 7 x64, Eclipse Mars, Java 1.7.
In order to implement this feature, I just copy/pasted the org.eclipse.wst.sse.ui.internal.handlers.ToggleLineCommentHandler class which in turn uses some internal classes among which CommentingStrategy.
I'm aware that non API class are marked x-friends in the plugin MANIFEST so that it gives an error at compile time which you can turn off, as Warning in my case...
But then at Runtime I'm getting the below error despite my plugin dependencies is correctly set :
And here is my MANIFEST :
So, is it absolutely forbidden to use eclipse internal classes ??
If so, how can I achieve my purpose which is simply textifying some characters in an XML file by surrounding them with <xsl:text> tag the same way as for the ToggleLineComment.
Thanks for your answers !
java.lang.NoClassDefFoundError: org/eclipse/wst/sse/ui/internal/comment/CommentingStrategy
at com.aksu.tools.tgs.handlers.TextifyHandler.processAction(TextifyHandler.java:112)
at org.eclipse.wst.sse.ui.internal.handlers.AbstractCommentHandler.execute(AbstractCommentHandler.java:62)
at org.eclipse.ui.internal.handlers.HandlerProxy.execute(HandlerProxy.java:295)
at org.eclipse.ui.internal.handlers.E4HandlerProxy.execute(E4HandlerProxy.java:90)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:56)
at org.eclipse.e4.core.internal.di.InjectorImpl.invokeUsingClass(InjectorImpl.java:252)
at org.eclipse.e4.core.internal.di.InjectorImpl.invoke(InjectorImpl.java:234)
at org.eclipse.e4.core.contexts.ContextInjectionFactory.invoke(ContextInjectionFactory.java:132)
at org.eclipse.e4.core.commands.internal.HandlerServiceHandler.execute(HandlerServiceHandler.java:152)
at org.eclipse.core.commands.Command.executeWithChecks(Command.java:493)
at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:486)
at org.eclipse.e4.core.commands.internal.HandlerServiceImpl.executeHandler(HandlerServiceImpl.java:210)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.executeCommand(KeyBindingDispatcher.java:286)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.press(KeyBindingDispatcher.java:507)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.processKeyEvent(KeyBindingDispatcher.java:558)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.filterKeySequenceBindings(KeyBindingDispatcher.java:378)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.access$0(KeyBindingDispatcher.java:324)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher$KeyDownFilter.handleEvent(KeyBindingDispatcher.java:86)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.filterEvent(Display.java:1266)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1112)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1137)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1122)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1164)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1160)
at org.eclipse.swt.widgets.Widget.wmChar(Widget.java:1581)
at org.eclipse.swt.widgets.Control.WM_CHAR(Control.java:4795)
at org.eclipse.swt.widgets.Canvas.WM_CHAR(Canvas.java:343)
at org.eclipse.swt.widgets.Control.windowProc(Control.java:4676)
at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:339)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:5050)
at org.eclipse.swt.internal.win32.OS.DispatchMessageW(Native Method)
at org.eclipse.swt.internal.win32.OS.DispatchMessage(OS.java:2549)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3767)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$4.run(PartRenderingEngine.java:1127)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:337)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1018)
at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:156)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:694)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:337)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:606)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:150)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:139)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:380)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:235)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:669)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:608)
at org.eclipse.equinox.launcher.Main.run(Main.java:1515)
at org.eclipse.equinox.launcher.Main.main(Main.java:1488)
Caused by: java.lang.ClassNotFoundException: org.eclipse.wst.sse.ui.internal.comment.CommentingStrategy cannot be found by com.aksu.tools.tgs_1.0.0
at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:439)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:352)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:344)
at org.eclipse.osgi.internal.loader.ModuleClassLoader.loadClass(ModuleClassLoader.java:160)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 60 more