ARROW-6241: [Java] Failures on master

Related to [ARROW-6241](https://issues.apache.org/jira/browse/ARROW-6241).

Closes apache#5090 from tianchen92/ARROW-6241 and squashes the following commits:

ced6ed5 <tianchen> ARROW-6241:  Failures on master

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>

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;