Create Namer and Emitter on codegen start.

pkg/dart2js_incremental/lib/caching_compiler.dart

Index: pkg/dart2js_incremental/lib/caching_compiler.dart
diff --git a/pkg/dart2js_incremental/lib/caching_compiler.dart b/pkg/dart2js_incremental/lib/caching_compiler.dart
index 8bf1c64127c6f97ce89ecba7785f6568d2b4797d..44d56a6c21e69f6552dbd7780b98b32e7eb34c74 100644
--- a/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -68,16 +68,6 @@ Future<CompilerImpl> reuseCompiler(
             environment: environment));
     backend = compiler.backend;
 
-    full.Emitter emitter = backend.emitter.emitter;
-
-    // Much like a scout, an incremental compiler is always prepared. For
-    // mixins, classes, and lazy statics, at least.
-    emitter
-        ..needsClassSupport = true
-        ..needsMixinSupport = true
-        ..needsLazyInitializer = true
-        ..needsStructuredMemberInfo = true;
-
     Uri core = Uri.parse("dart:core");
 
     return compiler.setupSdk().then((_) {