add a few classes and methods to the classpath

Add java.lang.CharSequence, java.util.AbstractSet,
java.util.AbstractCollection, Collections.unmodifiableSet,
 System.getProperty(String,String), etc.

classpath/java/lang/StringBuffer.java

@@ -10,7 +10,7 @@
 
 package java.lang;
 
-public class StringBuffer {
+public class StringBuffer implements CharSequence {
   private final StringBuilder sb;
 
   public StringBuffer(String s) {
@@ -116,4 +116,12 @@ public class StringBuffer {
   public synchronized String toString() {
     return sb.toString();
   }
+  
+  public String substring(int start, int end) {
+    return sb.substring(start, end);
+  }
+  
+  public CharSequence subSequence(int start, int end) {
+    return sb.subSequence(start, end);
+  }
 }