Support for required fields.

Change-Id: I560c7ca11b3d665eecafb528f3737b7e139ca9b0
Tested: on Linux and Windows.

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -242,6 +242,17 @@ public class FlatBufferBuilder {
         return vtableloc;
     }
 
+    // This checks a required field has been set in a given table that has
+    // just been constructed.
+    public void required(int table, int field) {
+        int table_start = bb.capacity() - table;
+        int vtable_start = table_start - bb.getInt(table_start);
+        boolean ok = bb.getShort(vtable_start + field) != 0;
+        // If this fails, the caller will show what field needs to be set.
+        if (!ok)
+            throw new AssertionError("FlatBuffers: field " + field + " must be set");
+    }
+
     public void finish(int root_table) {
         prep(minalign, SIZEOF_INT);
         addOffset(root_table);