Work around the fact that AssertionError(String message, Throwable cause) is a Java 1.7-only API.

src/main/java/org/elasticsearch/index/fielddata/plain/FSTBytesAtomicFieldData.java

@@ -112,7 +112,10 @@ public class FSTBytesAtomicFieldData implements AtomicFieldData.WithOrdinals<Scr
                 }
                 assert fstEnum.next() == null;
             } catch (IOException e) {
-                throw new AssertionError("Cannot happen", e);
+                // Don't use new "AssertionError("Cannot happen", e)" directly as this is a Java 1.7-only API
+                final AssertionError error = new AssertionError("Cannot happen");
+                error.initCause(e);
+                throw error;
             }
             this.hashes = hashes;
         }