Allow all different string-literals in map literals.

lib/compiler/implementation/scanner/listener.dart

Index: lib/compiler/implementation/scanner/listener.dart
diff --git a/lib/compiler/implementation/scanner/listener.dart b/lib/compiler/implementation/scanner/listener.dart
index 6b3184ee090686e2230521416db0a7fcacad3ffc..343bc5156a52e39edaf6b7f0e9caf4d6d76fb668 100644
--- a/lib/compiler/implementation/scanner/listener.dart
+++ b/lib/compiler/implementation/scanner/listener.dart
@@ -1213,8 +1213,8 @@ class NodeListener extends ElementListener {
   void endLiteralMapEntry(Token colon, Token endToken) {
     Expression value = popNode();
     Expression key = popNode();
-    if (key.asLiteralString() === null) {
-      recoverableError('expected a constant string', node: key);
+    if (key.asStringNode() === null) {
+      recoverableError('expected a string', node: key);
     }
     pushNode(new LiteralMapEntry(key, colon, value));
   }