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..ff4f0c67c38babe4d650f8904ad8d153a278125f 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -253,18 +253,7 @@ patch class Isolate {
       // The VM will invoke [_startIsolate] with entryPoint as argument.
       readyPort = new RawReceivePort();
       _spawnFunction(readyPort.sendPort, entryPoint, message);
-      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();
@@ -281,23 +270,34 @@ patch class Isolate {
       // The VM will invoke [_startIsolate] and not `main`.
       readyPort = new RawReceivePort();
       _spawnUri(readyPort.sendPort, uri.toString(), args, message);
-      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 String) {
+        // We encountered an error starting the new isolate.
+        completer.completeError(new IsolateSpawnException(
+            "Unable to spawn child isolate:\n${readyMessage}"));
+      } else 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 {
+        completer.completeError(new IsolateSpawnException(
+            "Internal error: received unexpected message $readyMessage"));
       }
-      return new Future<Isolate>.error(e, st);
     };
     return completer.future;
   }