dart2js: Use an abstract Name class for names in the generated JavaScript ast.

pkg/compiler/lib/src/ssa/nodes.dart

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index 575044550131e4c458ce57d17ba7733cdbf33712..3268ad49efa59c3cd21f346067cc266e59e733fa 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -211,6 +211,14 @@ class HGraph {
         compiler);
   }
 
+  HConstant addConstantStringFromName(js.Name name,
+                                      Compiler compiler) {
+    return addConstant(
+        new SyntheticConstantValue(SyntheticConstantKind.NAME,
+                                   js.quoteName(name)),
+        compiler);
+  }
+
   HConstant addConstantBool(bool value, Compiler compiler) {
     return addConstant(
         compiler.backend.constantSystem.createBool(value), compiler);