Allow all different string-literals in map literals.

lib/compiler/implementation/tree/nodes.dart

Index: lib/compiler/implementation/tree/nodes.dart
diff --git a/lib/compiler/implementation/tree/nodes.dart b/lib/compiler/implementation/tree/nodes.dart
index 71326fb607522bb2a342cc502a76c4d3aca8024f..2074ddc0bdcb282fa71d4dd06298783fd695ea64 100644
--- a/lib/compiler/implementation/tree/nodes.dart
+++ b/lib/compiler/implementation/tree/nodes.dart
@@ -141,6 +141,7 @@ class Node implements Hashable {
   Send asSend() => null;
   SendSet asSendSet() => null;
   Statement asStatement() => null;
+  StringNode asStringNode() => null;
   StringInterpolation asStringInterpolation() => null;
   StringInterpolationPart asStringInterpolationPart() => null;
   StringJuxtaposition asStringJuxtaposition() => null;
@@ -744,6 +745,8 @@ class StringQuoting {
 class StringNode extends Expression {
   abstract DartString get dartString();
   abstract bool get isInterpolation();
+
+  StringNode asStringNode() => this;
 }
 
 class LiteralString extends StringNode {