implement kernel -> ssa for literals

pkg/compiler/lib/src/kernel/kernel_visitor.dart

 // Copyright (c) 2016, 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.md file.
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/frontend/accessors.dart'
     show
         Accessor,
         IndexAccessor,
         NullAwarePropertyAccessor,
@@ skipping 187 matching lines @@
   final Map<CascadeReceiver, ir.VariableGet> cascadeReceivers =
       <CascadeReceiver, ir.VariableGet>{};
 
   final Map<ir.Node, Element> nodeToElement = <ir.Node, Element>{};
 
   ir.Node associate(ir.Node node, Element element) {
     nodeToElement[node] = element;
     return node;
   }
 
+  final Map<ir.Node, Node> nodeToAst = <ir.Node, Node>{};
+
   bool isVoidContext = false;
 
   KernelVisitor(this.currentElement, this.elements, this.kernel);
 
   KernelVisitor get sendVisitor => this;
 
   KernelVisitor get declVisitor => this;
 
   ir.TreeNode visitForValue(Expression node) {
     bool wasVoidContext = isVoidContext;
@@ skipping 673 matching lines @@
   }
 
   @override
   ir.Expression visitLiteralString(LiteralString node) {
     if (node.dartString == null) return new ir.InvalidExpression();
     return new ir.StringLiteral(node.dartString.slowToString());
   }
 
   @override
   ir.SymbolLiteral visitLiteralSymbol(LiteralSymbol node) {
-    return new ir.SymbolLiteral(node.slowNameString);
+    var result = new ir.SymbolLiteral(node.slowNameString);
+    nodeToAst[result] = node;
+    return result;
   }
 
   @override
   visitMetadata(Metadata node) {
     // Shouldn't be called. Metadata should already have been analyzed and
     // converted to a constant expression in the resolver.
     return internalError(node, "Metadata not handled as constant.");
   }
 
   @override
@@ skipping 1793 matching lines @@
       : this(null, true, node, initializers);
 
   accept(ir.Visitor v) => throw "unsupported";
 
   visitChildren(ir.Visitor v) => throw "unsupported";
 
   String toString() {
     return "IrFunction($kind, $isConstructor, $node, $initializers)";
   }
 }