Start isolates in a separate thread.

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 a98bb5a052f6d855e324ea0d53d6c70f1d7e4efc..bb3737159a0c5c13f52dda777906b26862bd822e 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -284,18 +284,7 @@ patch class Isolate {
       readyPort = new RawReceivePort();
       _spawnFunction(readyPort.sendPort, entryPoint, message,
                      paused, errorsAreFatal, onExit, onError);
-      Completer completer = new Completer<Isolate>.sync();
-      readyPort.handler = (readyMessage) {
-        readyPort.close();
-        assert(readyMessage is List);
-        assert(readyMessage.length == 2);
-        SendPort controlPort = readyMessage[0];
-        List capabilities = readyMessage[1];
-        completer.complete(new Isolate(controlPort,
-                                       pauseCapability: capabilities[0],
-                                       terminateCapability: capabilities[1]));
-      };
-      return completer.future;
+      return _spawnCommon(readyPort);
     } catch (e, st) {
       if (readyPort != null) {
         readyPort.close();
@@ -328,24 +317,36 @@ patch class Isolate {
                 errorsAreFatal, checked,
                 null, /* environment */
                 packageRootString, packagesList);
-      Completer completer = new Completer<Isolate>.sync();
-      readyPort.handler = (readyMessage) {
+      return _spawnCommon(readyPort);
+    } catch (e, st) {
+      if (readyPort != null) {
         readyPort.close();
-        assert(readyMessage is List);
-        assert(readyMessage.length == 2);
+      }
+      return new Future<Isolate>.error(e, st);
+    }
+  }
+
+  static Future<Isolate> _spawnCommon(RawReceivePort readyPort) {
+    Completer completer = new Completer<Isolate>.sync();
+    readyPort.handler = (readyMessage) {
+      readyPort.close();
+      if (readyMessage is List && readyMessage.length == 2) {
         SendPort controlPort = readyMessage[0];
         List capabilities = readyMessage[1];
         completer.complete(new Isolate(controlPort,
                                        pauseCapability: capabilities[0],
                                        terminateCapability: capabilities[1]));
-      };
-      return completer.future;
-    } catch (e, st) {
-      if (readyPort != null) {
-        readyPort.close();
+      } else if (readyMessage is String) {
+        // We encountered an error while starting the new isolate.
+        completer.completeError(new IsolateSpawnException(
+            'Unable to spawn isolate: ${readyMessage}'));
+      } else {
+        // This shouldn't happen.
+        completer.completeError(new IsolateSpawnException(
+            "Internal error: unexpected format for ready message: "
+            "'${readyMessage}'"));
       }
-      return new Future<Isolate>.error(e, st);
-    }
+    };
     return completer.future;
   }