implement opassign, fix bugs in pre/postfix, introduce a let* helper

lib/runtime/dart/isolate.js

Index: lib/runtime/dart/isolate.js
diff --git a/lib/runtime/dart/isolate.js b/lib/runtime/dart/isolate.js
index 980e27278f54b19c006f9545b2f1cb773aee2e70..30f391d6d36617910178daa69eb8f6b2814a9e5c 100644
--- a/lib/runtime/dart/isolate.js
+++ b/lib/runtime/dart/isolate.js
@@ -70,34 +70,34 @@ var isolate;
     }
     [_pause](resumeCapability) {
       let message = new core.List(3);
-      dart.dsetindex(message, 0, "pause");
-      dart.dsetindex(message, 1, this.pauseCapability);
-      dart.dsetindex(message, 2, resumeCapability);
+      message[core.$set](0, "pause");
+      message[core.$set](1, this.pauseCapability);
+      message[core.$set](2, resumeCapability);
       this.controlPort.send(message);
     }
     resume(resumeCapability) {
       let message = new core.List(2);
-      dart.dsetindex(message, 0, "resume");
-      dart.dsetindex(message, 1, resumeCapability);
+      message[core.$set](0, "resume");
+      message[core.$set](1, resumeCapability);
       this.controlPort.send(message);
     }
     addOnExitListener(responsePort) {
       let message = new core.List(2);
-      dart.dsetindex(message, 0, "add-ondone");
-      dart.dsetindex(message, 1, responsePort);
+      message[core.$set](0, "add-ondone");
+      message[core.$set](1, responsePort);
       this.controlPort.send(message);
     }
     removeOnExitListener(responsePort) {
       let message = new core.List(2);
-      dart.dsetindex(message, 0, "remove-ondone");
-      dart.dsetindex(message, 1, responsePort);
+      message[core.$set](0, "remove-ondone");
+      message[core.$set](1, responsePort);
       this.controlPort.send(message);
     }
     setErrorsFatal(errorsAreFatal) {
       let message = new core.List(3);
-      dart.dsetindex(message, 0, "set-errors-fatal");
-      dart.dsetindex(message, 1, this.terminateCapability);
-      dart.dsetindex(message, 2, errorsAreFatal);
+      message[core.$set](0, "set-errors-fatal");
+      message[core.$set](1, this.terminateCapability);
+      message[core.$set](2, errorsAreFatal);
       this.controlPort.send(message);
     }
     kill(priority) {
@@ -109,21 +109,21 @@ var isolate;
       if (pingType === void 0)
         pingType = Isolate.IMMEDIATE;
       let message = new core.List(3);
-      dart.dsetindex(message, 0, "ping");
-      dart.dsetindex(message, 1, responsePort);
-      dart.dsetindex(message, 2, pingType);
+      message[core.$set](0, "ping");
+      message[core.$set](1, responsePort);
+      message[core.$set](2, pingType);
       this.controlPort.send(message);
     }
     addErrorListener(port) {
       let message = new core.List(2);
-      dart.dsetindex(message, 0, "getErrors");
-      dart.dsetindex(message, 1, port);
+      message[core.$set](0, "getErrors");
+      message[core.$set](1, port);
       this.controlPort.send(message);
     }
     removeErrorListener(port) {
       let message = new core.List(2);
-      dart.dsetindex(message, 0, "stopErrors");
-      dart.dsetindex(message, 1, port);
+      message[core.$set](0, "stopErrors");
+      message[core.$set](1, port);
       this.controlPort.send(message);
     }
     get errors() {