ARROW-6241: [Java] Failures on master

Author: tianchen92

java/vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -28,6 +28,7 @@
 import org.apache.arrow.memory.BaseAllocator;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.memory.OutOfMemoryException;
+import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.AddOrGetResult;
 import org.apache.arrow.vector.BitVectorHelper;
 import org.apache.arrow.vector.BufferBacked;

java/vector/src/main/java/org/apache/arrow/vector/complex/NonNullableStructVector.java

@@ -26,6 +26,7 @@
 import java.util.Map;
 
 import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.DensityAwareVector;
 import org.apache.arrow.vector.FieldVector;
 import org.apache.arrow.vector.ValueVector;