Allow all different string-literals in map literals.

lib/compiler/implementation/tree/nodes.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface Visitor<R> {
   R visitBlock(Block node);
   R visitBreakStatement(BreakStatement node);
   R visitCatchBlock(CatchBlock node);
   R visitClassNode(ClassNode node);
   R visitConditional(Conditional node);
@@ skipping 123 matching lines @@
   Modifiers asModifiers() => null;
   NamedArgument asNamedArgument() => null;
   NodeList asNodeList() => null;
   Operator asOperator() => null;
   ParenthesizedExpression asParenthesizedExpression() => null;
   Return asReturn() => null;
   ScriptTag asScriptTag() => null;
   Send asSend() => null;
   SendSet asSendSet() => null;
   Statement asStatement() => null;
+  StringNode asStringNode() => null;
   StringInterpolation asStringInterpolation() => null;
   StringInterpolationPart asStringInterpolationPart() => null;
   StringJuxtaposition asStringJuxtaposition() => null;
   SwitchCase asSwitchCase() => null;
   SwitchStatement asSwitchStatement() => null;
   Throw asThrow() => null;
   TryStatement asTryStatement() => null;
   TypeAnnotation asTypeAnnotation() => null;
   Typedef asTypedef() => null;
   TypeVariable asTypeVariable() => null;
@@ skipping 583 matching lines @@
     return mapping[(raw ? 1 : 0) + index * 2 + (quote === $SQ ? 8 : 0)];
   }
 }
 
 /**
   * Superclass for classes representing string literals.
   */
 class StringNode extends Expression {
   abstract DartString get dartString();
   abstract bool get isInterpolation();
+
+  StringNode asStringNode() => this;
 }
 
 class LiteralString extends StringNode {
   final Token token;
   /** Non-null on validated string literals. */
   final DartString dartString;
 
   LiteralString(this.token, this.dartString);
 
   LiteralString asLiteralString() => this;
@@ skipping 883 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send !== null) return f(send.arguments.head);
 }