Create Namer and Emitter on codegen start.

tests/compiler/dart2js/type_representation_test.dart

Index: tests/compiler/dart2js/type_representation_test.dart
diff --git a/tests/compiler/dart2js/type_representation_test.dart b/tests/compiler/dart2js/type_representation_test.dart
index 2b16579f390cd3389c905bf02999cbb9813bac32..bb1280d3136ee7430e1e78804f183a6e87cef099 100644
--- a/tests/compiler/dart2js/type_representation_test.dart
+++ b/tests/compiler/dart2js/type_representation_test.dart
@@ -41,6 +41,8 @@ void testTypeRepresentations() {
       m9(int a, String b, {List<int> c, d}) {}
       m10(void f(int a, [b])) {}
       """).then((env) {
+        var closedWorld = env.compiler.closeResolution();
+        env.compiler.backend.onCodegenStart(closedWorld);
         TypeRepresentationGenerator typeRepresentation =
             new TypeRepresentationGenerator(env.compiler);