Allow Symbols in switch and maps

tests/language/symbol_literal_test.dart

Index: tests/language/symbol_literal_test.dart
===================================================================
--- tests/language/symbol_literal_test.dart	(revision 42183)
+++ tests/language/symbol_literal_test.dart	(working copy)
@@ -12,6 +12,14 @@
 
 var check = foo; // Indirection used to avoid inlining.
 
+testSwitch(Symbol s) {
+  switch(s) {
+    case #abc: return 1;
+    case const Symbol("def"): return 2;
+    default: return 0;
+  }
+}
+
 main() {
   check(const Symbol("a"), #a);
   check(const Symbol("a"), #
@@ -28,6 +36,11 @@
   check(const Symbol("=="), # ==);
   check(const Symbol("a.toString"), #a.toString);
 
+  Expect.equals(1, testSwitch(#abc));
+
+  const m = const <Symbol, int>{#A:0, #B:1};
+  Expect.equals(1, m[#B]);
+
   // Tries to call the symbol literal #a.toString
   Expect.throws(() => #a.toString(), (e) => e is NoSuchMethodError); /// 01: static type warning
 }