dart2js: use IdGenerator for all id generation

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 25893e2348476fe71009b234637b503de8606b09..07a453a9c7e2eb3d00ce437e3193ad31eba222b3 100644
--- a/pkg/compiler/lib/src/resolution/class_hierarchy.dart
+++ b/pkg/compiler/lib/src/resolution/class_hierarchy.dart
@@ -299,8 +299,11 @@ class ClassResolverVisitor extends TypeDefinitionVisitor {
     String superName = supertype.name;
     String mixinName = mixinType.name;
     MixinApplicationElementX mixinApplication =
-        new UnnamedMixinApplicationElementX("${superName}+${mixinName}",
-            element.compilationUnit, compiler.getNextFreeId(), node);
+        new UnnamedMixinApplicationElementX(
+            "${superName}+${mixinName}",
+            element.compilationUnit,
+            compiler.idGenerator.getNextFreeId(),
+            node);
     // Create synthetic type variables for the mixin application.
     List<DartType> typeVariables = <DartType>[];
     int index = 0;