Consistently use setters to set event handlers in native APIs.

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
diff --git a/runtime/bin/process_impl.dart b/runtime/bin/process_impl.dart
index dbe6981eb1d12033e1981acda799b697939ee128..9dcff2a2f9e67a87749761b47ab660ec50273fc8 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -146,7 +146,7 @@ class _Process implements Process {
     _closed = true;
   }
 
-  void setExitHandler(void callback(int exitCode)) {
+  void set exitHandler(void callback(int exitCode)) {
     if (_closed) {
       throw new ProcessException("Process closed");
     }