Make Enqueuer a pure interface.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index c4c357c72854cee67dee6e24b46e8dad5e1af788..7c3342da5e022b4e8d903ace2745501f9bd74b1c 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -321,7 +321,7 @@ class JavaScriptBackend extends Backend {
    * The generated code as a js AST for compiled methods.
    */
   Map<Element, jsAst.Expression> get generatedCode {
-    return compiler.enqueuer.codegen.generatedCode;
+    return codegenEnqueuer.generatedCode;
   }
 
   FunctionInlineCache inlineCache = new FunctionInlineCache();
@@ -1564,6 +1564,8 @@ class JavaScriptBackend extends Backend {
     }
   }
 
+  CodegenEnqueuer get codegenEnqueuer => compiler.enqueuer.codegen;
+
   CodegenEnqueuer createCodegenEnqueuer(Compiler compiler) {
     return new CodegenEnqueuer(compiler, createItemCompilationContext,
         const TreeShakingEnqueuerStrategy());