Implement spawnUri from dart:isolate. This function allows us to

tests/isolate/isolate.status

Index: tests/isolate/isolate.status
===================================================================
--- tests/isolate/isolate.status	(revision 7714)
+++ tests/isolate/isolate.status	(working copy)
@@ -5,13 +5,17 @@
 [ $runtime == vm ]
 isolate2_negative_test: Skip  # Need to resolve correct behaviour.
 serialization_test: Skip  # tests frog-specific serialization code
-v2_spawn_uri_test: Fail # not implemented
+v2_spawn_uri_test: Fail # test uses a ".js" suffix that is bogus on vm.
 v2_compute_this_script_browser_test: Skip # browser specific test
 
 [ ($compiler == none || $compiler == dart2js) && $runtime == drt ]
 serialization_test: Skip  # tests frog-specific serialization code
 v2*: Skip # new api for isolates not implemented in the V_m yet.
 
+[ $compiler == frog ]
+v2_spawn_uri_vm_test: Fail # test uses a ".dart" suffix that only works in vm.
+v2_spawn_uri_vm_negative_test: Skip # ditto above.
+
 [ $compiler == frog && $runtime == d8 ]
 v2_spawn_uri_test: Fail # not implemented for d8
 v2_compute_this_script_browser_test: Skip # browser specific test
@@ -64,6 +68,8 @@
 v2_spawn_function_test: Fail, OK # Leg does not support APIv2
 v2_spawn_uri_test: Fail, OK # Leg does not support APIv2
 v2_unresolved_ports_test: Fail, OK # Leg does not support APIv2
+v2_spawn_uri_vm_test: Fail # test uses a ".dart" suffix that only works in vm.
+v2_spawn_uri_vm_negative_test: Skip # ditto above.
 constructor_test: Fail, OK # Leg does not support subclassing Isolate
 
 [ $compiler == dart2js && $runtime == none ]