Improve error handling in the process library.

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 b121d2cece06d22bdd2004086834d22c20286273..a868599e85e2a27b217fa4276c505d86954ca786 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -11,14 +11,24 @@ class _ProcessStartStatus {
 class _Process implements Process {
 
   _Process(String path, List<String> arguments) {
+    if (path is !String) {
+      throw new ProcessException("Path is not a String: $path");
+    }
     _path = path;
-    {
-      int len = arguments.length;
-      _arguments = new ObjectArray<String>(len);
-      for (int i = 0; i < len; i++) {
-        _arguments[i] = arguments[i];
+
+    if (arguments is !List) {
+      throw new ProcessException("Arguments is not a List: $arguments");
+    }
+    int len = arguments.length;
+    _arguments = new ObjectArray<String>(len);
+    for (int i = 0; i < len; i++) {
+      var arg = arguments[i];
+      if (arg is !String) {
+        throw new ProcessException("Non-string argument: $arg");
       }
+      _arguments[i] = arguments[i];
     }
+
     _in = new _Socket._internal();
     _out = new _Socket._internal();
     _err = new _Socket._internal();