Add Isolate.current getter, returning the current isolate as an Isolate object.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index 1313609002de2d77afab21a30c262f499610bcf0..eb88de61913ad044aeae36d32dfe8c608ae980b1 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -17,6 +17,8 @@ patch class Capability {
 
 class _CapabilityImpl implements Capability {
   factory _CapabilityImpl() native "CapabilityImpl_factory";
+
+  String toString() => "Capability#";
 }
 
 patch class RawReceivePort {
@@ -245,6 +247,10 @@ void _startIsolate(SendPort parentPort,
 }
 
 patch class Isolate {
+  static final _currentIsolate = _getCurrentIsolate();
+
+  /* patch */ static Isolate get current => _currentIsolate;
+
   /* patch */ static Future<Isolate> spawn(
       void entryPoint(message), var message, { bool paused: false }) {
     // `paused` isn't handled yet.
@@ -368,4 +374,14 @@ patch class Isolate {
   /* patch */ void removeErrorListener(SendPort port) {
     throw new UnsupportedError("removeErrorListener");
   }
+
+  static Isolate _getCurrentIsolate() {
+    List portAndCapabilities = _getPortAndCapabilitiesOfCurrentIsolate();
+    return new Isolate(portAndCapabilities[0],
+                       pauseCapability: portAndCapabilities[1],
+                       terminateCapability: portAndCapabilities[2]);
+  }
+
+  static List _getPortAndCapabilitiesOfCurrentIsolate()
+      native "Isolate_getPortAndCapabilitiesOfCurrentIsolate";
 }