Support general expressions as keys in literal maps.

sdk/lib/_internal/compiler/implementation/scanner/listener.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/listener.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
index 1e8f36a4a958ff37f0a079ba6898acff20779727..dc912fb6dc637361375f5330459e94aaddb676c0 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
@@ -1622,9 +1622,6 @@ class NodeListener extends ElementListener {
   void endLiteralMapEntry(Token colon, Token endToken) {
     Expression value = popNode();
     Expression key = popNode();
-    if (key.asStringNode() == null) {
-      recoverableError('expected a string', node: key);
-    }
     pushNode(new LiteralMapEntry(key, colon, value));
   }