Add more tests and argument check to Process.sleep

sdk/lib/io/process.dart

Index: sdk/lib/io/process.dart
diff --git a/sdk/lib/io/process.dart b/sdk/lib/io/process.dart
index 8ceb41d9e7d16a61d188b22901800cdfadccc91d..1c9fddb985eef94ae8147aaf5bf6b6f010d6c014 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -46,7 +46,11 @@ set exitCode(int status) {
  * in a isolate while it is blocked in a [sleep] call.
  */
 void sleep(Duration duration) {
-  _ProcessUtils._sleep(duration.inMilliseconds);
+  int milliseconds = duration.inMilliseconds;
+  if (milliseconds < 0) {
+    throw new ArgumentError("sleep: duration cannot be negative");
+  }
+  _ProcessUtils._sleep(milliseconds);
 }
 
 /**