WIP: Compute constant expressions in resolution.

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 3dca32bb4515634d3d6e46465927ddeec5b7a12c..83a8355651c5f9bbeb325d8d135b8895fc3b1e90 100644
--- a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
@@ -9,7 +9,7 @@ import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../constant_system_dart.dart';
 import '../dart_types.dart';
-import '../elements/elements.dart' show ClassElement;
+import '../elements/elements.dart' show ClassElement, FieldElement;
 import '../tree/tree.dart' show DartString, LiteralDartString;
 import 'js_backend.dart';
 
@@ -193,6 +193,7 @@ class JavaScriptConstantSystem extends ConstantSystem {
       const JavaScriptBinaryArithmeticOperation(const MultiplyOperation());
   final negate = const JavaScriptNegateOperation();
   final not = const NotOperation();
+  final notEqual = const NotEqualsOperation();
   final shiftLeft =
       const JavaScriptBinaryBitOperation(const ShiftLeftOperation());
   final shiftRight = const JavaScriptShiftRightOperation();
@@ -267,6 +268,19 @@ class JavaScriptConstantSystem extends ConstantSystem {
         compiler.backend.typeImplementation.computeType(compiler.resolution));
   }
 
+  @override
+  ConstantValue createSymbol(Compiler compiler, String text) {
+    InterfaceType type = compiler.symbolClass.rawType;
+    ConstantValue argument = createString(new DartString.literal(text));
+    Map<FieldElement, ConstantValue> fields = <FieldElement, ConstantValue>{};
+    compiler.symbolImplementationClass.forEachInstanceField(
+        (ClassElement enclosingClass, FieldElement field) {
+      fields[field] = argument;
+    });
+    assert(fields.length == 1);
+    return new ConstructedConstantValue(type, fields);
+  }
+
   // Integer checks don't verify that the number is not -0.0.
   bool isInt(ConstantValue constant) => constant.isInt || constant.isMinusZero;
   bool isDouble(ConstantValue constant)