Allow all different string-literals in map literals.

lib/compiler/implementation/compile_time_constants.dart

Index: lib/compiler/implementation/compile_time_constants.dart
diff --git a/lib/compiler/implementation/compile_time_constants.dart b/lib/compiler/implementation/compile_time_constants.dart
index 4d47b96d62fb344250774770fae086da3d5475c0..d660d9e1694c2c339138bdead8ae01a1561743a9 100644
--- a/lib/compiler/implementation/compile_time_constants.dart
+++ b/lib/compiler/implementation/compile_time_constants.dart
@@ -739,7 +739,7 @@ class CompileTimeConstantEvaluator extends AbstractVisitor {
          link = link.tail) {
       LiteralMapEntry entry = link.head;
       Constant key = evaluate(entry.key);
-      if (!key.isString() || entry.key.asLiteralString() === null) {
+      if (!key.isString() || entry.key.asStringNode() === null) {
         MessageKind kind = MessageKind.KEY_NOT_A_STRING_LITERAL;
         compiler.reportError(entry.key, new ResolutionError(kind, const []));
       }