Align synthetic mixin names with fasta.

pkg/compiler/lib/src/resolution/class_hierarchy.dart

Index: pkg/compiler/lib/src/resolution/class_hierarchy.dart
diff --git a/pkg/compiler/lib/src/resolution/class_hierarchy.dart b/pkg/compiler/lib/src/resolution/class_hierarchy.dart
index 0d9b50cfa0dac064c6ac80c4bb704f652d22ce50..8bce3cae35d50e9e78fc7d7f3d5b2ff58bb4a483 100644
--- a/pkg/compiler/lib/src/resolution/class_hierarchy.dart
+++ b/pkg/compiler/lib/src/resolution/class_hierarchy.dart
@@ -447,7 +447,7 @@ class ClassResolverVisitor extends TypeDefinitionVisitor {
           if (index != -1) {
             name = name.substring(0, index);
           }
-          name = "$name&${mixin.name}$signature";
+          name = "_$name&${mixin.name}$signature";
           mixinElement = mixinApplicationClasses[name];
           if (mixinElement != null) return mixinElement.thisType;