Change IllegalArgumentException to ArgumentError.

runtime/bin/path_impl.dart

Index: runtime/bin/path_impl.dart
diff --git a/runtime/bin/path_impl.dart b/runtime/bin/path_impl.dart
index 42ef0c2a69030fefb239b40383514d7b85069a88..9c7e43825e96a84a26c092c59c7adcfe4f8d816b 100644
--- a/runtime/bin/path_impl.dart
+++ b/runtime/bin/path_impl.dart
@@ -58,7 +58,7 @@ class _Path implements Path {
 
   Path join(Path further) {
     if (further.isAbsolute) {
-      throw new IllegalArgumentException(
+      throw new ArgumentError(
           "Path.join called with absolute Path as argument.");
     }
     if (isEmpty) {