dart2js: Move most of the code_emitter_task code into the program-builder.

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 0050ab069aced1cc08463683a4c43b739c678776..343a94b3783fbaacc27f11d3ad6c449a6b90908e 100644
--- a/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -139,16 +139,6 @@ Future<Compiler> reuseCompiler(
         ..clearCspPrecompiledNodes()
         ..elementDescriptors.clear();
 
-    backend.emitter
-        ..nativeClassesAndSubclasses.clear()
-        ..outputContainsConstantList = false
-        ..neededClasses.clear()
-        ..outputClassLists.clear()
-        ..outputConstantLists.clear()
-        ..outputStaticLists.clear()
-        ..outputStaticNonFinalFieldLists.clear()
-        ..outputLibraryLists.clear();
-
     backend
         ..preMirrorsMethodCount = 0;