Change Process.start to return a future that completes with a

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 47ccd8d8ff386aad0014c91a082ef125a392ca57..0e33800064e516f9ac454bae2722232cd7515a80 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -14,12 +14,26 @@ class _Process extends NativeFieldWrapperClass1 implements Process {
   static Future<ProcessResult> run(String path,
                                    List<String> arguments,
                                    [ProcessOptions options]) {
-    return new _NonInteractiveProcess._start(path, arguments, options)._result;
+    return new _NonInteractiveProcess(path, arguments, options)._result;
   }
 
-  _Process.start(String path,
-                 List<String> arguments,
-                 ProcessOptions options) {
+  static Future<Process> start(String path,
+                               List<String> arguments,
+                               ProcessOptions options) {
+    Completer completer = new Completer();
+    _Process process = new _Process(path, arguments, options);
+    process._onStart = () {
+      process.onError = null;
+      completer.complete(process);
+    };
+    process.onError = (e) {
+      process.onError = null;
+      completer.completeException(e);
+    };
+    return completer.future;
+  }
+
+  _Process(String path, List<String> arguments, ProcessOptions options) {
     if (path is !String) {
       throw new ArgumentError("Path is not a String: $path");
     }
@@ -269,10 +283,6 @@ class _Process extends NativeFieldWrapperClass1 implements Process {
     _onError = callback;
   }
 
-  void set onStart(void callback()) {
-    _onStart = callback;
-  }
-
   void _reportError(e) {
     if (_onError != null) {
       _onError(e);
@@ -304,9 +314,9 @@ class _Process extends NativeFieldWrapperClass1 implements Process {
 // _NonInteractiveProcess is used to implement the Process.run
 // method.
 class _NonInteractiveProcess {
-  _NonInteractiveProcess._start(String path,
-                                List<String> arguments,
-                                ProcessOptions options) {
+  _NonInteractiveProcess(String path,
+                         List<String> arguments,
+                         ProcessOptions options) {
     _completer = new Completer<ProcessResult>();
     // Extract output encoding options and verify arguments.
     var stdoutEncoding = Encoding.UTF_8;
@@ -329,10 +339,10 @@ class _NonInteractiveProcess {
     }
 
     // Start the underlying process.
-    _process = new _Process.start(path, arguments, options);
+    _process = new _Process(path, arguments, options);
 
     // Make sure stdin is closed.
-    _process.onStart = _process.stdin.close;
+    _process._onStart = _process.stdin.close;
 
     // Setup process error handling.
     _process.onError = (e) => _completer.completeException(e);