fix bug in isolate code, reenable isolate tests

lib/isolate/frog/isolateimpl.dart

Index: lib/isolate/frog/isolateimpl.dart
diff --git a/lib/isolate/frog/isolateimpl.dart b/lib/isolate/frog/isolateimpl.dart
index 267ccb4f523dbdef54f06d7fde4b6f7c881bce25..876a2b06f9634b5025a497375b9fe997e2feb46a 100644
--- a/lib/isolate/frog/isolateimpl.dart
+++ b/lib/isolate/frog/isolateimpl.dart
@@ -171,7 +171,7 @@ class _IsolateContext {
    * Run [code] in the context of the isolate represented by [this]. Note this
    * is called from JavaScript (see $wrap_call in corejs.dart).
    */
-  void eval(Function code) {
+  Dynamic eval(Function code) {
     var old = _globalState.currentContext;
     _globalState.currentContext = this;
     this._setGlobals();