Reexport start_timer, stop_timer

Make start_timer validate its arguments.

Ref: discussion in #10885

Author: jiahao

NEWS.md

@@ -738,7 +738,7 @@ Deprecated or removed
     `index > length(collection)+1` ([#7373]).
 
   * No longer exported from `Base`:
-    * `start_timer`, `stop_timer`, `start_reading`, `stop_reading`, `start_watching` ([#10885]).
+    * `start_reading`, `stop_reading`, `start_watching` ([#10885]).
 
 Julia v0.2.0 Release Notes
 ==========================

base/exports.jl

@@ -1171,6 +1171,8 @@ export
     serialize,
     skip,
     skipchars,
+    start_timer,
+    stop_timer,
     takebuf_array,
     takebuf_string,
     truncate,

base/stream.jl

@@ -502,6 +502,9 @@ function _uv_hook_asynccb(async::AsyncWork)
 end
 
 function start_timer(timer::Timer, timeout::Real, repeat::Real)
+    timeout ≥ 0 || throw(ArgumentError("timer cannot have negative timeout of $timeout seconds"))
+    repeat ≥ 0 || throw(ArgumentError("timer cannot repeat $repeat times"))
+
     associate_julia_struct(timer.handle, timer)
     preserve_handle(timer)
     ccall(:uv_update_time,Void,(Ptr{Void},),eventloop())