Create 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 39826f656a469ed179aab3353180142ac6db3dd9..ffd2b587eec279463bff895a55f15a55265d8de1 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -256,13 +256,19 @@ patch class Isolate {
       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]));
+        if (readyMessage is String) {
+          // We encountered an error starting the new isolate.
+          completer.completeError(new IsolateSpawnException(
+              "Unable to spawn child isolate:\n${readyMessage}"));
+        } else {
+          assert(readyMessage is List);

[Cutch, 2014/08/27 18:18:43]

These asserts will only trigger in checked mode. Should you do:

if (readyMessage is String) {
  completer.completeError...
} else if ((readyMessage is List) && (readyMessage.length == 2)) {
  completer.complete
} else {
  completer.completeError...
}

?

[turnidge, 2014/08/27 19:28:59]

Done.

On 2014/08/27 18:18:43, Cutch wrote:

+          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;
     } catch (e, st) {
@@ -284,13 +290,19 @@ patch class Isolate {
       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]));
+        if (readyMessage is String) {
+          // We encountered an error starting the new isolate.
+          completer.completeError(new IsolateSpawnException(
+              "Unable to spawn child isolate:\n${readyMessage}"));
+        } else {
+          assert(readyMessage is List);
+          assert(readyMessage.length == 2);
+          SendPort controlPort = readyMessage[0];
+          List capabilities = readyMessage[1];
+          completer.complete(new Isolate(controlPort,

[Cutch, 2014/08/27 18:18:43]

Duplicate code from above? Perhaps you can extract this entire closure into a
utility function.

[turnidge, 2014/08/27 19:28:59]

Extracted common code into new function spawnCommon.

+                                         pauseCapability: capabilities[0],
+                                         terminateCapability: capabilities[1]));
+        }
       };
       return completer.future;
     } catch (e, st) {