public class TestViewer extends ViewPart {
	public void createPartControl(Composite parent) {
		Button b = new Button(parent, SWT.PUSH);
		b.setText("Click to test");
		
		b.addSelectionListener(new SelectionListener() {

			@Override
			public void widgetSelected(SelectionEvent e) {
				fire();
			}

			@Override
			public void widgetDefaultSelected(SelectionEvent e) {
			}
		});
	}

	private void fire() {
		IPath path = new Path("ASDF");	// <----- It works
		new MySite();	// Exception occurred while loading a class from ClassLoader
	}
}

import org.eclipse.team.internal.core.target.Site;

public class MySite extends Site {
...
    public TargetProvider newProvider(IPath intrasitePath) throws TeamException {
		return new MyTargetProvider(this, intrasitePath);
    }
...
}