Use correct type on evaluated literal symbols.

pkg/compiler/lib/src/js_backend/constant_system_javascript.dart

Index: pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
index f3d2b4c98e072432885e863e6fbecc5aaf57e05e..410d1c86bbb1fd8c2ac83fe00c8a00a78eb4e98b 100644
--- a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
@@ -349,11 +349,12 @@ class JavaScriptConstantSystem extends ConstantSystem {
   @override
   ConstantValue createSymbol(Compiler compiler, String text) {
     // TODO(johnniwinther): Create a backend agnostic value.
-    InterfaceType type = compiler.coreTypes.symbolType;
+    JavaScriptBackend backend = compiler.backend;
+    ClassElement symbolClass = backend.helpers.symbolImplementationClass;
+    InterfaceType type = symbolClass.rawType;

[Siggi Cherem (dart-lang), 2016/05/26 19:52:15]

shouldn't symbolClass.rawType and coreTypes.symbolType be the same?

I guess symbolType comes from the unpatched Symbol, and symbolClass would be
Symbol.implementation? Should we anyways have an invariant in our system that
the type returned by a class element and it's implementation is the same
(cls.rawType == cls.implementation.rawType)?

[Johnni Winther, 2016/05/27 08:51:24]

It's worse. `coreTypes.symbolType` is the one defined in dart:core. The
`backend.helpers.symbolImplementationClass` is defined in dart:_internal. We
have

In dart:core :

class Symbol {
  external const factory Symbol(String name);
}

in core_patch.dart :

import 'dart:_internal' as internal;

@patch
class Symbol {
  @patch
  const factory Symbol(String name) = internal.Symbol;
}

and in dart:_internal :

import 'dart:core' as core;

class Symbol implements core.Symbol {
  final String name;

  const Symbol(String name);
}

[Siggi Cherem (dart-lang), 2016/05/27 17:27:08]

yikes - should we change the patch file to directly contain the implementation
instead?

[Johnni Winther, 2016/05/30 07:58:23]

I think we want to share the implementation between dart2js and VM (and
Dartino?). Don't know why; I guess Peter knows.

     ConstantValue argument = createString(new DartString.literal(text));
     Map<FieldElement, ConstantValue> fields = <FieldElement, ConstantValue>{};
-    JavaScriptBackend backend = compiler.backend;
-    backend.helpers.symbolImplementationClass.forEachInstanceField(
+    symbolClass.forEachInstanceField(
         (ClassElement enclosingClass, FieldElement field) {
       fields[field] = argument;
     });