Fix dart2js for switches in async code with only awaits in the key.

pkg/compiler/lib/src/js/rewrite_async.dart

Index: pkg/compiler/lib/src/js/rewrite_async.dart
diff --git a/pkg/compiler/lib/src/js/rewrite_async.dart b/pkg/compiler/lib/src/js/rewrite_async.dart
index c2863c5d895c9e32fc00dcc6819e9818b200cf4d..65408295e6f8bb56e49fc736df550a4b5c8143f6 100644
--- a/pkg/compiler/lib/src/js/rewrite_async.dart
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -2412,7 +2412,10 @@ class PreTranslationAnalysis extends js.NodeVisitor<bool> {
   @override
   bool visitSwitch(js.Switch node) {
     loopsAndSwitches.add(node);
-    bool result = visit(node.key);
+    // If the key has an `await` expression, do not transform the
+    // body of the switch.
+    visit(node.key);
+    bool result = false;
     for (js.SwitchClause clause in node.cases) {
       if (visit(clause)) result = true;
     }