Hook async/await compiler into pub and use it.

sdk/lib/_internal/pub/lib/src/command/run.dart

Index: sdk/lib/_internal/pub/lib/src/command/run.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/run.dart b/sdk/lib/_internal/pub/lib/src/command/run.dart
index 9881b6de19fa273d0f41797ff47e3ba4a3c4b5ed..45c0931a3fb324b029f31a71b4b8a713c1bf2adb 100644
--- a/sdk/lib/_internal/pub/lib/src/command/run.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/run.dart
@@ -19,7 +19,7 @@ class RunCommand extends PubCommand {
       "NOTE: We are currently optimizing this command's startup time.";
   String get usage => "pub run <executable> [args...]";
 
-  Future onRun() {
+  Future onRun() async {
     if (commandOptions.rest.isEmpty) {
       usageError("Must specify an executable to run.");
     }
@@ -36,7 +36,8 @@ class RunCommand extends PubCommand {
       executable = components[1];
     }
 
-    return runExecutable(this, entrypoint, package, executable, args)
-        .then(flushThenExit);
+    var exitCode = await runExecutable(this, entrypoint, package, executable,
+        args);
+    await flushThenExit(exitCode);
   }
 }